Clean up mal_import.rb

Rebased commits:
* Rewrite get_characters and get_series methods
* Remove ResourceFetcher since we spoof UA now
* Move mal_import.rb to lib/
* Change mal_id to external_id
* Add switch for @media, adding initial Manga support
* Handle featured characters within MAL instead of returning mal_ids[]
* Handle empty synopsis case
Add create_or_update_from_hash methods

Rebased commits:
* Fix mass assignment things to expose what we will need
* Anime#get_metadata_from_mal --> Anime.create_or_update_from_hash
* Add create_or_update_by_hash to Casting and Character and Person