Introduce a 'link_request' directive.

See #185.
Add a better hint to the link_prefix docstring
Always add the link_prefix
Removes link_prefix caching, adds broken test
Fixes link_prefix tests (and a bug in request.view)
Cache link_prefix per app and request.
Add suggested cosmetic changes
Merge pull request #234 from seantis/master

Introduce a 'link_request' directive.