Normalize filepath in ICE_Files::theme_file_to_url().

Another Windows issue!

`ICE_Files::theme_file_to_url()` needs to normalize the filepath before attempting to convert the theme file to a hyperlink.

On Windows installs, the URL was not correct and was causing the following issue as reported on CBOX.org:
http://commonsinabox.org/groups/help-support/forum/topic/possible-cbox-theme-options-bug-resulting-from-upgrade-to-wordpress-3-6/#post-3264

I added a commit to CBOX Theme to address this:
https://github.com/cuny-academic-commons/cbox-theme/commit/3c0dae65f12368ba03b930e443164aaf6ab36329