Add song contributions

This includes performers, composers, arrangers, and lyricists.
Add kinds to artists

The attribute can be set to either a person or group. It is named `kind`
instead of `type` to prevent having to work around Rails' STI
implementation.

This commit also introduces `LP::Enumable`, an ActiveRecord module to
handle enumerated types.
Use Enumable for contribution kinds
Add translations to models

This introduces `LP::Translatable`, an ActiveRecord module that handles
model attribute translations. It does basic caching of translation
models but may be invalidated if translations are modified directly.

To prevent having to use `attr_accessible`, `strong_parameters` replaces
it for mass-assignment. This is the default method in Rails 4 and is
compatible with it.