Better wildcard fall-through for assets

Only serve the wildcard if:

  * GET or HEAD is being called.
  * There are no "." characters in the URL, OR the url extension starts
    with "htm", OR the url extension is over 5 chars long.

This will cause JavaScript and other assets to fail with 404 errors as normally expected.

Ref:
https://github.com/stefanpenner/ember-app-kit/commit/02882c7da3e3745606c595ea5566b52bff7567a2#commitcomment-3963307