Link to active fork for ODBC backend

It took me quite some time to find if and where the ODBC backend was maintained.
I found (on djangoproject.com):
http://code.google.com/p/django-pyodbc/ (last commit about 3 years ago)
then:
https://github.com/avidal/django-pyodbc avidal fork.
then:
https://github.com/aurorasoftware/django-pyodbc/ aurorasoftware version which has avidal improvements merged.

Avidals version now links to https://github.com/aurorasoftware/django-pyodbc/ which is also the version installed through PIP.
Merge pull request #1209 from tax/master

Updated link to active project for ODBC backend