New context() API

This makes the API much easier to use, and resembles the raven-ruby implementation. Users can continually merge context (generally within their middleware), and simply need to call context.clear() to refresh it.

The following APIs are added as part of this changeset:

```
Client.context -- the accessible context instance
>>> Client.context.get()

Client.context.clear() -- flush the current context
>>> Client.context.clear()

Client.user_context() -- a shortcut to set the user interface
>>> Client.user_context({'email': 'foo@example.com'})

Client.tags_context() -- add additional tags to context (or overwrite an existing tag)
>>> Client.tags_context({'key': 'value'})

Client.extra_context() -- add additional data to context (or overwrite an existing key in the data)
>>> Client.extra_context({'key': 'value'})
```