fixed rich comparison methods for session

(reposted from #7 because of mishaps with git filter-branch)

Porting Session Saver to Python 3 removed the ```__cmp__``` method for comparing objects and replaced them with the rich comparison methods like ```__lt__```. However, during the port, ```__eq__``` wasn't added to the Session class, which meant that identically named sessions weren't considered equal. This meant that sessions would never be changed, leading them to be duplicated:
![session_duplicate](https://cloud.githubusercontent.com/assets/1497826/3867388/c4e6e85a-1ffc-11e4-871e-e4f1bffd09a5.png)
This adds the ```__eq__``` method in, and the ```__ne__``` method just for completeness.