Fixed #90 - Added fields django_id and django_ct.

- Fields are not fully indexed because they are not text.
- Added test of order_by django_id.
- Simplified code for updating index.
Refactored code.

Avoided starting TermGenerator for each object being indexed.
Refactored xapian_backend.update.

Removed repeated code.
Fixed #112 - more_like_this raises InvalidIndexError for unindexed instance.

This doesn't happen if silently_fail is True, in which case returns
an empty result. Added test to cover both cases.
Merge pull request #121 from jorgecarleitao/issue112

Fixed #112 - more_like_this raises InvalidIndexError for unindexed instance.

This doesn't happen if silently_fail is True, in which case it returns
an empty result. Added test to cover both cases.
Merge pull request #122 from jorgecarleitao/issue90

Fixed #90 - Adds support to fields "django_id" and "django_ct".