PEP8 + Django conventions + etc
Move choices to Message model
Added tests for messagelog
Added initial migration for messagelog
Add docs for messagelog
Changes based on comments from @dpoirier
Update release notes for messagelog
Merge remote-tracking branch 'rapidsms/release-0.12.0' into docs/messagelog

Conflicts:
	rapidsms/contrib/messagelog/templates/messagelog/index.html
Merge pull request #221 from rapidsms/docs/messagelog

Test and document contrib.messagelog