Add custom content-type/mimetype

Content-types are defined as subclasses of restfulgrok.contenttype.ContentType. Take look at its docs, and the sourcecode of restfulgrok.contenttype.JsonContentType:

class JsonContentType(ContentType):
    """
    JSON content type. Implements both loads and dumps.
    """
    mimetype = 'application/json'
    extension = 'json'
    description = json_description

    @classmethod
    def dumps(cls, pydata, view=None):
        try:
            return json.dumps(pydata, indent=2)
        except TypeError, e:
            raise ContentTypeDumpError(str(e))
        except ValueError, e:
            raise ContentTypeDumpError(str(e))

    @classmethod
    def loads(cls, rawdata, view=None):
        try:
            return json.loads(rawdata)
        except TypeError, e:
            raise ContentTypeLoadError(str(e))
        except ValueError, e:
            raise ContentTypeDumpError(str(e))