Improvements for xapian-haystack

These are fixes and improvements provided for the xapian-haystack project.

Fixing untrustable os.path.exists():

https://github.com/notanumber/xapian-haystack/pull/175

Add DJANGO_CT, DJANGO_ID, ID to be used with __exact internally:

https://github.com/notanumber/xapian-haystack/pull/232