Make Castings polymorphic
Add Castings for staff and characters in Manga
Add Castings to full manga serializer and add template/view for it

It's one-line changes in a lot of files, really
Properly pluralize "Manga" in Ember
Fix styling of manga characters to match the cast lists of anime pages
Fix serializer to properly serialize relationship for castings
Import image for manga characters from MAL
Merge pull request #116 from NuckChorris/poly-castings

Make Casting-Series association polymorphic