Output exception message if value is compared to NULL in search.
Test case for comparison to NULL with generic operators.
Fixes typo in docstring.
PEP8
Merge pull request #299 from mjfroehlich/null_comparison_error_message

Show error message if search query tests for NULL using comparison operators
Records changes due to previous commits.
Catch validation exceptions in relations during update
Merge pull request #338 from adelq/master

Catch validation exceptions in relations during update
skip_unless now shows one skip for each test method if class was provided
unregistering Flask-SQLAlchemy session signal handlers to avoid of mess with sessions
improve python3 compatibility
some coding style fixes