Update metadata.py
BIT types can be NULL according to http://technet.microsoft.com/en-us/library/ms177603.aspx
Update README.md

inspectdb and subsequent queries on a 2012 database worked for me without errors or warnings.
broke introspection
partial revert failed
reverted to working, simple NullBooleanField fix
Revert "broke introspection"

This reverts commit c1e27ace16e16db11054ce3cf5becda9ad963fca.
comment about ill-advised conversion of TextField to CharField
Merge branch 'master' of github.com:hobsonlane/django-pyodbc
Merge pull request #32 from hobsonlane/master

`bit` column types should default to `NullBooleanField` types in Django