create topics model
create tones model
add tags method to story model
add tags to story representer
add uniqueness constraint to topics and tags so that story cannot have repeated tags
change story representer test to use hardcoded tag names
use wont_be :method syntax for topic and tone validation tests
remove NPR topic ID stuff
add sorting to tags method
Merge pull request #50 from PRX/tags

Tags, hurray! :fireworks: