Responde requisições POST.
Factory for controllers based on the HTTP method.

A controller is responsible for creating a Response object which will be
delivered to the user. Get responses may be delivered as plain/text or
as JSON, the responses were previously cerated by a "CreateResponse"
class which only served the GET controller created here so this moves
the strategy form that class into a single private method to the Get
controller (as it is not usefull anywhere else).

Currently controllers are responsible for creating the Response object
delivered to the user and the Post controller doesn't do anything at
all (tests are failing to indicate that). Ideally controllers should
create the base response object and later, based on the Accept header of
the request, the response should be properly serialized to the end
user by a different object.

This adds some @TODO tags with the refactors in mind.