Solution to clean child theme urls

Note: This pull request is a response to issue #461

This approach makes minimal changes and maintains the use of `$wp_rewrite`.

The results are not perfect; the child theme urls are rewritten from `/wp-content/themes/somechildtheme/*.*` to `/child/*.*`.

The word `child` could be replaced with a more incognito string, but I think in the end something more involved would be a prettier solution. See http://github.com/retlehs/roots/issues/461#issuecomment-7159191 for my thoughts on that. This is a first step for now.

**UPDATE**

I now have it doing some basic extension checking so that urls look like `/child/css/style.css` and `/child/js/...`. Images do not get the `/img/` directory prefixed/appended.

It would also be possible to flip the strings so that the urls would look like `/css/child/style.css` -- that's a matter of preference.

**UPDATE 2**

I have applied the rewrite conditions and rules proposed by @Foxaii in #461. The `/child/` and directory (`/js/`/`/css/`) prefixes have been removed in favor of checking for the file first in the child theme before looking in roots.