New pod urls

This proposal is partial work toward #176.

Moves two urls:
* `/pod/Foo` (previously `/module/Foo`)... good for `L<>` pod links: matches any `documentation`
* `/pod/release/$author/$release/@path` (previously `/module/$author/$release/@path`)

Adds:
* `/pod/distribution/$name/@path` for version independent pod links (would close #805).

Still to be done:
* disambiguation page
* better canonical url generation (if indexed module use /pod/$doc otherwise use new `/pod/distribution/@blah`)

Thoughts?