Use `static_path`

[tilgovi]

Moved from hypothesis/h#1672

This should help certain build scenarios where the output should be relocatable and the app domain is the same as the asset domain.

If webassets.base_url is absolute, request.static_path will still be absolute, so that shouldn't be an issue.

Reported by @ikreymer at http://list.hypothes.is/archive/dev/2014-11/0000027.html

[erichulser]

+1, I just had to hack in this change to support https.

[tilgovi]

Conveniently, I suddenly have reason to do this as of the past week. If you'd like to submit a pull request, though, don't hesitate!

[erichulser]

I just hacked the installed version, so I don't have a pull request to offer.