ObjLibraryClient cleanup + misc fixes

This PR contains a complete cleanup of ```gi.libraries``` methods, moving implementations and documentation from ```ObjLibraryClient``` class to ```Library```.

It also contains:
- small documentation enhancements;
- the rename of wrongly named method ```get_current_history()``` to ```get_most_recently_used_history()```;
- a workaround for Requests 2.4 which was making TravisCI builds fail.