pep8 violation fixes
Correct pylint violations
Add test case for invalid handler URLs
A common default set_many implementation
Merge pull request #45 from edx/sarina/violation-removal

Eliminate all pep8 and most pylint warnings
Merge pull request #47 from edx/dhm/set_many

A common default set_many implementation
Revert the naming of default view `context` argument
Merge pull request #48 from edx/sarina/revert-_context

Revert the naming of default view `context` argument
Add tests that fail with mutable types
Handle saving of mutable fields correctly.

Add `MUTABLE` flag to `ModelType`s so we know what is and isn't mutable.
Default behavior is Mutable.
Merge pull request #53 from edx/sarina/mutable-bulk-save

Sarina/mutable bulk save
Add diff-quality reports to cibuild
Merge pull request #54 from edx/sarina/build-diff-quality

Add diff-quality reports to cibuild
Merge master (with a few tweaks)
Make XBlock.__repr__ resilient to errors in the field storage.