Fixed null pointer exception saving list
Fixed bug while saving attached objects
[maven-release-plugin] prepare release persistence-0.9.20
[maven-release-plugin] prepare for next development iteration
Make sure we commit only if there's actually an active transaction
[maven-release-plugin] prepare release persistence-0.9.21
[maven-release-plugin] prepare for next development iteration
Changed way in which on upgrade works
 - DbOpenHelper.onUpgrade method is now final. You will now have to override a new method called onUpgradeDatabase.
 - Updated copyright
 - Added methods to get the table name and column names (SQLHelper class)
Merge pull request #3 from casidiablo/develop

Updated master before next release