create format model

I created a format model and added it to the story.tags method. Basically the same deal as the tones and topics. The only change I made to the FORMAT_NAMES list was to combine all the fundraising categories into one. I didn't make any attempt to change existing format names in the database to fit the new list -- should I?
I'm not sure I like that you're testing by basically re-implementing, and the sorting is a thing to worry about in my experience.

Maybe make use of must_include (http://ruby-doc.org/stdlib-1.9.3/libdoc/minitest/spec/rdoc/MiniTest/Expectations.html#method-i-must_include) ?