Custom content negotiation

[multimeric]

• Allow an API to define a dictionary of converter functions for different content types, that are run before and after the JSON API schema sees the data. For example:

  api = Api(blueprint=bp, response_renderers={
      'text/csv': render_csv
  }, request_parsers={
      'text/csv': parse_csv
  })

• Two tests added in a separate file
• Fixes Non-JSON MIME types #170 and/or fixes how do you override Content-Type / Accept headers? #13

[coveralls]

Coverage decreased (-2.3%) to 88.518% when pulling 5a7938c on TMiguelT:content-negotiation into b44bc08 on miLibris:master.