Add schema generation (first-pass)

[richmolj]

JsonapiCompliable::Schema.generate([Array, Of, Resource, Classes]) will generate a jsonapi-specific schema. A backwards-compatibility check will be added in a subsequent commit.

If using Rails, no need to pass an array of classes. We will eager load everything and grab all relevant descendants of Resource.

Notes:

• Filters can now limit operators with only: and except:
• Resources are now associated with endpoints. A subsequent commit will ensure resources cannot be called from an endpoint if not whitelisted. This will tie into automatic link generation.
• We now require all relevant keys when registering a custom type.
• This is a first-pass and will be altered as we roll out 1.0

[richmolj]

@wadetandy