change FilterSet to invoke form errors and skip filtering on invalid data, returning empty queryset with strict mode
change default value for strict to True, and hopefully made the code for filtering with invalid values clearer (also removed _SENTINEL)
- need to differentiate between actual 'None' value and a ValidationError
- grab order_by value in for loop, avoids double lookup and eliminates need for get_value function
fix a few tests due to defaulting strict = True
Merge branch 'develop' into nkryptic-issue-84
Merge pull request #108 from apollo13/issue-84

Issue 84