Start adding mixpanel support; #686

This adds their js snippet and starts logging a couple events. Once I
hit distinct_id I realized I really do want #680 first, though. Shelving
on a branch for now.
Start converting ORM to username; #680
Merge branch 'master' into username
Start branch.sql to fix username/id; #287, #680

This breaks pretty much everything, as promised:

    https://twitter.com/whit537/status/319426649479344128
    https://twitter.com/whit537/status/319493424917143552

:)
Get homepage working with username; #287, #680
Rename %participant_id dir to %username; #287
Merge branch 'master' into username
Big changeover from id to username; #287

I sort of did this one with the blast shield down. I haven't even run
the tests yet. The sorts of things I changed:

 - SQL {elsewhere,exchanges}.participant_id => .participant
 - SQL participants.id => .username
 - ORM {user,participant,self}.id => .username
Started working through test failures; #287
Merge branch 'master' into username

Conflicts:
	www/%username/history/index.html
	www/assets/%version/gittip.css
Fix some tests re: participant model; #287
Test suite is passing! #287
Add at least one test for id; #287
Fix a regression in the history page; #287
Fix regressions in Delpan report; #287
Fix regressions in stats; #287
Fix regression in on/confirm.html; #287