Merge pull request #2 from Plypy/master

Refactoring and Clean-up for Code Style and Fix Typo
Merge pull request #3 from Plypy/master

Session exception list: temporary fix for issue ID-12
fix comment typo
Bump to version 1.5.0
Merge remote-tracking branch 'upstream/master' into new-db
rearrange the model files
added locked property and updated tests
added some comments
added todo file
integrated new datamodel