Allow namespace resolving

Will allow namespaces to be targeted for resolving:

`other@adapter:post` resolves to `other/adapters/post`

This is different than the Custom Type Prefixes in
https://github.com/stefanpenner/ember-resolver/pull/16 as that feature
only allows one chance to access the module. But if you want to have a
default module path and if that fails lookup a similar path in another
prefix the Custom Type Prefixes won't work and you must target the
namespaces specifically in the resolving path.

Also add support for namespace lookups from view helpers:

`view:other@post` normailzes to `other@view:post`

Includes regression test for looking up views