add get() to the expression context

You can now use `get(key)` in an organ expression, which makes it
possible to access dict keys (or object attributes) with characters that
are invalid local variable names. For instance:

```python
>>> import organ
>>> organ.expression("get('foo bar') == 'baz'")({'foo bar': 'baz'})
True
```

This bumps the version to 0.4.3.