BoundLog matcher implemented

Based on discussion in #633, `BoundLog` matcher has been implemented that can be used in tests by calling 
```python
self.assertEqual(self.log, matches(IsBoundWith(a=2, b=3))
```
This is based on http://testtools.readthedocs.org/en/latest/for-test-authors.html#writing-your-own-matchers.

Thanks to @lvh for providing the awesome partial kwargs extraction code :)