Merge branch 'release/0.2.8' into develop
Clean up readme.
Harsher slugification.
Add a new column to the persons model to keep track of rank.
Add failing test for cached ranking.
Isolate those tests a little more.
Add a __repr__ to make debugging tests easier.
Recalculate and cache rank each time a new assertion is awarded.
Expose a Person's rank in __json__.
Accept a notification callback and invoke it when rank changes.
Remove spurious backslash.
Simplify that adjust_ranks call.
Add a test for pre-existing users with no cached rank.
Additionally, send the badge award message too.  Because that just makes sense.
Introduce timelimits to the leaderboard maker.

This code and test will end up powering these two features:

- fedora-infra/tahrir#163
- fedora-infra/tahrir#174
Also.. watch out for these.
Revert "Also.. watch out for these."

This reverts commit 46c9e7389d92ec4abf04ff96d026ad01de501202.
Change that fedmsg topic from "update" to "advance" (since it will always be advancing, and we never emit messages for rank slipping).
Add a comment on the meaning of the 'start' and 'stop' parameters.
Add a docstring for _adjust_ranks.