Added tabs between object form and generic content form on PUT/PATCH form

Some extra behaviour to `BrowsableAPIRenderer` to handle PATCH form.
Added PATCH button on generic content PUT form.
Tabs between object form and generic content form on PUT/PATCH form wich are
both allways visible now.

Fix #570
Refs #591
Added tests for PATCH form in the Browsable API
POST form using new form.html template
Added tabs between object form and generic content form on POST form
Minimal forms appearance improvements
Merge branch 'browsable_api_patch' of https://github.com/wronglink/django-rest-framework into generic-form-input
Polishing touches to #663
Tweak TemplateHTMLRenderer docs.
Merge pull request #670 from tomchristie/generic-form-input

PUT or PATCH raw data (eg. json) from Browseable API