changes:

Hi, 
I made a bit changes as following:
1 replace NotImplementedException by Exception to avoid compile error
2 add Mongo class, make hhvm-mongo-driver looks like php-mongo-drive the same exactly
3 fix MongoCursor addOption bug
All changes was tested, please take a look, thanks.
This class was purposefully excluded, as the Mongo class has been deprecated since 1.3.0 for the PHP driver.