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 change isn't discussed at all in the commit message. Why are you disabling this method here?

Also, NotImplementException (or `throw_not_implemented()` in HHVM 3.2+) is absolutely preferable to MongoCursorException in this case. If a method is not implemented for some reason, we should be following HHVM's convention for reporting that to the user. MongoCursorException and other MongoException child classes have specific meanings already and should not be used for this purpose.