The model now supports badges with multiple categories.

The approach probably isn't very efficient, but it was the easiest
way to add the functionality given the tight deadline.

This doesn't add any route/view support for the new functionality,
however. That comes next.

Soon, #143 will be crushed.
added routes/views for multi-category badges.
added a TODO; we define dedupe() which seems exactly like _.uniq().
normalize category information before badge validation.
Merge pull request #148 from toolness/issue-143-multi-category

Badges can have multiple STEAM categories