Implement a 'format registry', for responding differently depending on Accept headers

Here, in combination with the following line we are adding defaultOutputFormat to be an acceptable parameter. As the default should be considered after all other parameters it should have the lowest q priority q=0.

so alternately:
<pre>
            default_params = AcceptParameters(
                ContentType(self.defaultOutputFormat, params='q=0'))

            acceptable = [ default_params ]
            acceptable += [  AcceptParameters(ContentType(x)) for x in self._outputFormatsPreference ]
</pre>