Add API views, serializers, filters for Protocol and ProtocolType

[jathanism]

• Added a new database migration to add ProtocolType.site since required_attributes are site-specific, so must be ProtocolType.
• Replaced ProtocolType.name unique constraint with unique_together constraint on site and name fields (Uniqueness enforced at site boundary).
• Moved ProtocolType.required_attributes validation from Protocol.clean_type() to Protocol.set_attributes() (which overloads Resource.set_attributes()
• Updated Protocol.clean_type() to validate that the incoming type matches the site.
• Resource.save() (abstract base) now optionally takes a valid_attributes argument that gets passed to .set_attributes()
• Fixed a bug in nsot.api.serializers.NaturalKeyRelatedField that was causing site-level-limiting in related fields to not work correctly.

[nickpegg]

Looks pretty good! Only a couple of comments.

Standard disclaimer: anything marked with suggestion: is entirely optional

[narJH27]

Looks good to me!