Add support for explicitly converting items to another.
Moved item override and fit porting to services.

Also sprinkled static methods around Fit service and other PEP8 goodness because why not.
Improved fitting window import; revert addition of staticmethod's in fit service
Fix #106
Merge pull request #97 from blitzmann/issue-67

Fixes #67
Unknown for DB items are fetched as None; make sure service doesn't fail in this case
Merge branch 'master' into singularity