Add a converter for boolean values

[marcogiusti]

If a numeric column (tinyint in my case) is mapped as a BooleanField, Django sometime report a wrong value that is always True. This because it checks for the type (...value is False...). The driver should handle this case and convert from a numeric value to a boolean.

[OskarPersson]

This is hopefully fixed in https://github.com/ESSolutions/django-mssql-backend/pull/17