Move segment assignment serverside

To get away from reloading the page with JS and all of the ugliness
that that entails, segment assignment is now done serverside and the
segment cookie now contains the name of the segment instead of an int
between 1 and 100.
Safer returning of segment cookie value

Instead of just returning the segment cookie value, check against the
condition names which were defined in the A/B test and return that
condition name.
Merge pull request #2 from bbrooks/serverside-assignment

Move segment assignment server-side