Fixes #11 - Mounter can read theme_assets symlinked on file system

This PR handles the case where the site symlinks in its theme assets. This could be useful in the case where the user is working on multi-sites and wants to share code among the sites (CSS, JavaScript, etc).

Before, Wagon wouldn't push these changes to the site because it couldn't follow the symlinks to find the theme assets.

This is my first contribution to Locomotive, and I'm not as familiar with the conventions and test habits of the site, so hopefully I came close.

What else would it take to help you guys be comfortable with the pull?