Clean up some pre-existing formatting.
Remove unused things.
Rename DbModel to KvsFieldData.
Implement InheritanceFieldData
Use XBlock xml serialization and deserialization

XModules continue to use their own interface for xml, but provide an
adaptor that makes the the XBlock interface available.

[LMS-179]
Prevent unicode data from being injected into a Location using the _replace function
Merge pull request #2129 from cpennington/xblocks-xml-import-export

XBlock Xml Serialization/Deserialization