separate spec-generation from mojo-ising

[mohawk2]

This is a further step towards separating out the OpenAPI spec. Any further and it will clash with #26.

Next steps as far as I can see would be:

• allow the supplying of an OpenAPI spec, documenting the (current) limitations of that
• separate out the editor into probably Yancy::Controller::Yancy::Editor (I confess I think the naming of Y::C::Y::API is Ok)

[coveralls]

Coverage increased (+0.06%) to 98.69% when pulling 1360c8a on mohawk2:openapi-separate into 7f8050e on preaction:master.

[preaction]

Looks good to me, thanks!