FIxed issue 15, added support for version overwrite.

Hi there, thanks for taking the djangopypi project forward from all the many unfinished clones :) Whilst I'm not much of a django fan this is certainly the most complete non-reference pypi implementation out there.
 
I've fixed the problem with the underlying media files not being deleted. Also, I've added the user option ALLOW_VERSION_OVERWRITE to enable clients to upload the same distribution more than once, this is a regex pattern that matches on the version number. 

The Sphinx docs have also been updated to outline the new settings.