Add headers pagination mixin for generic api view or viewsets.

This implements a simple way to remove a pagination metadata from response body. It helps to use a list json response as is, in javascript frameworks (without previous adaptation process).

I don't know if location of mixin is correct. Also, I don't know if it should be on rest-framework. But if you consider it useful for include into rest-framework, I will add doc and tests.

This pull-request is slightly similar to #1167 but implement a more generalist way to put metadata on headers instead of populate a body. In my opinion, put Link header with all data encoded as html/xml is making dificult to parse than simple value.