Appending to key does not work

Hello,
I've noticed that using `.append()` on a PickleShare Key does not represent the changes that were made.

# Example
Command Line 1:
```
>>> db = PickleShareDB('C:\\Users\\NULL\\AppData\\Local\\Temp\\test')
>>> db['stack'] = []
>>> db['stack']
[]
```
Command Line 2:
```
>>> db = PickleShareDB('C:\\Users\\NULL\\AppData\\Local\\Temp\\test')
>>> db['stack']
[]
```
Command Line 1:
```
>>> db['stack'].append("Hello World!")
>>> db['stack']
['Hello World!']
```
Command Line 2:
```
>>> db['stack']
[]
```

In fact, I added my own debugging, and the `.append()` does not even trigger the `__setitem__` function of the `PickleShareDB` class.
```
>>> from pickleshare import *
>>> db = PickleShareDB('test')
This is the debugged version.
>>> db['testvalue'] = 1234567
Set item called with key:testvalue and value:1234567
>>> db['stack'] = []
Set item called with key:stack and value:[]
>>> db['stack'].append("Hello")
```

# Fix
Since the `__setitem__` magic method only triggers when the value of a key is set, or updated, it does not trigger when you `.append() `something.
Also, there is no magic method that triggers when an `.append()` happens
So I suggest adding an `.append()` function:
```
def append(self, key, value):
currentValue = self[key]
currentValue.append(value)
updatedValue = currentValue
self[key] = updatedValue
```
# Test
Command Line 1:
```
>>> db = PickleShareDB('C:\\Users\\NULL\\AppData\\Local\\Temp\\test')
>>> db['stack'] = []
>>> db['stack']
[]
```
Command Line 2:
```
>>> db = PickleShareDB('C:\\Users\\NULL\\AppData\\Local\\Temp\\test')
>>> db['stack']
[]
```
Command Line 1:
```
>>> db.append('stack', 'Hello World!')
>>> db['stack']
['Hello World!']
```
Command Line 2:
```
>>> db['stack']
['Hello World!']
```

It would be nice if you could overwrite the default `.append()` function, like `db['stack'].append(0)` but since every `db['key']` is not an instance of the class PickleShareDB but a list, I don't want to mess with it :D