Implement track/stream

* Add Streamer class
* Use FactoryGirl for factories
* Add sha256 field to tracks
* Add mime_type field to tracks

features/step_definitions/playlists_step.rb

@@ -1,3 +1,3 @@
 Given /^a playlist named "([^"]*)"$/ do |name|
-  Playlist.create!(:name => name)
+  Factory.create(:playlist, :name => name)
 end