Normalize filepath in ICE_Files::theme_file_to_url().

Fixes issues with ICE_Component::locate_file_url() not returning the
proper file URL on Windows installs.

Specifically, on the CBOX Theme Options page, when attempting to upload an
image, media-upload.js was throwing a 404 because of the wrong file URL.

See:
http://commonsinabox.org/groups/help-support/forum/topic/possible-cbox-theme-options-bug-resulting-from-upgrade-to-wordpress-3-6/#post-3264