Add EventListener.on_query_completions and View.match_selector API docs
Completions enhancements
Exclude non-indexed overview.rst from the config

This will prevent the warning about this file not being included in any
toctree.
Update installation instructions and add a "contributing" section
Step through the first few doc pages and do style adjustments
Add a simple index for the API docs

I wanted Sphinx to auto generate this for me but apparently it didn't want to
so I resorted to do stuff manually, by hand. YES, I AM REALLY DISAPPOINT!