Adding override of API nickname for codegen.

Swagger has a nickname filed specified in the 1.1 standard. It is used
to generate an alternate name for code generators to use for that api.
This patch provides a way to override the generated nickname with one
specified in the grape documentation.

Example:
      desc(
        'Get all the cloud profiles in the system',
        :nickname => 'getAllCloudProfiles',
      )
Add list[] and void type for responseClass

* Generate a void responseClass if no entity is specified on a given
endpoint.

* Added the ability to specify a list[] of an entity as the responseClass
of a given endpoint.

responseClass: "list[PetEntity]"
  VS
responseClass: "PetEntity"