API Design - Request for feedback

Hi everyone, I've been working on `v2` for a while now, and it's getting close to a point where I can start making alpha releases!

However, reworking this has been a mostly one-man effort, and I'd really like some input on how these changes are going to suit everyone else (after all, you're probably using this library more than I am 😁). Additionally, if you need me to elaborate on something, or explain the reasoning behind these API choices, I'd be more than happy to!

You can also use this issue as a way of requesting features, if there's something, important or not, we're missing!

A quick primer of major changes before we begin:
- No Python 2 support
- Type hints
- Better listening system (no more error-prone subclassing)
- Cleaner way of interracting with threads
- `client.changeThreadColor(color, thread_id=thread_id)` -> `thread.set_color(color)`
- `async`/`await`, backed by `asyncio` - Not yet implemented, follow #411 for more

### Interracting with threads
Digging a bit further into the details, I'd like input on the changed API when interracting with threads. Previously, you'd call a method in `Client` with a thread ID and a `ThreadType`. This was pretty error prone, and involved a lot of extra typing. Now you simply create a `Group`/`User` object (or more often, fetch it from somewhere), and call methods on that.

An example:
```python
# Old
import fbchat
client = fbchat.Client("", "")
print("Own id: {}".format(client.uid))

client.send(fbchat.Message(text="Hi me!"), thread_id=client.uid, thread_type=fbchat.ThreadType.USER)

client.send(fbchat.Message(text="Hi group!"), thread_id="", thread_type=fbchat.ThreadType.GROUP)

# New
import fbchat
session = fbchat.Session.login("", "")
print("Own id: {}".format(session.user.id))

session.user.send_text("Hi me!")

group = fbchat.Group(session=session, id="")
group.send_text("Hi group!")
```

Note also how we have pretty clear separation of concerns (`Session` handles logging in, `Group` handles interracting with the group).

### Listening
Regarding listening, you'd previously override `Client`, and provide a custom implementation of an `onX` method. This was pretty error prone, and made it difficult for me to add new features. Now you create a `Listener` object, and register methods based on type hints.
**How this is going to work exactly is very much up for discussion, so please comment on this!**

An example:
```python
# Old
import fbchat

class CustomClient(fbchat.Client):
def onMessage(self, author_id, message, thread_id, thread_type, **kwargs):
# If you're not the author, echo
if author_id != self.uid:
self.send(fbchat.Message(text=message), thread_id=thread_id, thread_type=thread_type)

client = CustomClient("", "")
client.listen()

# New
import fbchat
session = fbchat.Session.login("", "")
listener = fbchat.Listener(session=session, ...) # TODO: Addition parameters

@listener.register
def on_message(event: fbchat.MessageEvent):
# If you're not the author, echo
if event.author.id != session.user.id:
event.thread.send_text(event.message.text)

listener.run()
```

A few notes before we round this off:
- I probably won't be adding caching, the requirements for this changes vastly across applications, so I'd rather provide a solid foundation that people can build this upon
- See https://github.com/carpedm20/fbchat/issues/396 for the wishlist, and feature requests
- If you want to see more examples of how the API has changed, you can look at the difference between the [`v1` examples](https://github.com/carpedm20/fbchat/tree/v1/examples) and the [current WIP examples](https://github.com/carpedm20/fbchat/tree/master/examples)

Thanks for your time 👍