Loader: fix for py3k, allow override via URL fragment

Python 3 implements `HTTPMessage` using `email.message.Message`, while Python 2 uses `mimetools.Message`. Updated code to use `get_content_type()` instead of `gettype()` for Python 3.

This is untested on Python 3, as I cannot get Kivy to run under Python 3.3 or 3.4 and the Installation on Linux page still lists 'Python >= 2.7 and Python < 3' as a requirement.

Also added a check for URL fragment to allow override of the file type by appending '#.ext' to the request.