FR: Implement context manager to mock time.

I just stumbled upon [dmc](https://github.com/rhettg/dmc), a different library for handling dates and times in Python. With this library you are able to [mock the current time](https://github.com/rhettg/dmc#testing). It could be like this:

```
with arrow.mock('2013-05-11T21:23:58.970460+00:00'):
assert arrow.now() == arrow.get('2013-05-11T21:23:58.970460+00:00')
```

This feature would be very useful for unit testing methods which use `arrow.now()` or a related method.

Start by locating the implementations of arrow.now() and arrow.get(), then review the surrounding tests for date and time behavior. Define the arrow.mock() context manager around the example in the issue, and verify that arrow.now() returns the mocked timestamp only within the context.

Written by the indexing model from the issue text.