Allow custom checkers via the WATCHMAN_CHECKS setting; defaults to checking databases and caches
Implements #3
Code review tweaks

* Rename `only_check` GET param to `check`
* Rename `get_checkers` and `paths_to_checkers` to `get_checks` and
  `paths_to_checks`
* Rename `available_checkers` to `available_checks`
PEP8 fixes
Update docs and other tweaks

* Add kezabelle to contributors
* Fix `make docs` command
* Update docs
  * Custom checks
  * How to run a subset of checks
  * New Default checks section
  * Check in modules and watchman sections
Merge pull request #15 from mwarkentin/settings_checkers

Allow custom checkers via the WATCHMAN_CHECKS setting