Check mergeability more frequently
Fix a bug on removing an assignee
Do not assume the build results to be successful it it is not provided
Make some commands work only when the bot is mentioned
Properly check if the user is one of the reviewers
Initial work.

Things that work:
  * Installation (`pip install -e .` or `python setup.py install`)
  * `homu` command. `homu -c CONF [-s {restart,stop}]`

TODO: get the script to save start the process and save the PID.
Hooked up homu/scripts/homu to homu.main

Doesn't quite work yet, but getting there.