Inverse catalog filter.
- Catalog.filter() can now return inverted results, if "inverse" is passed
  as an argument.
- Code reorganization for easier readability
Merge remote-tracking branch 'upstream/master' into inverse_filter

Conflicts:
	obspy/core/event.py
Fix code formatting.
Make "inverse" a kwarg.
Update documentation for core.event.Catalog.filter
Test for inverse catalog filtering.
Updated CHANGELOG for inverse catalog filtering.
Merge remote-tracking branch 'upstream/master' into inverse_filter

Conflicts:
	CHANGELOG.txt
Merge pull request #525 from claudiodsf/inverse_filter

Inverse filtering of catalogs