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.
NotImplementedException appears to have been removed in facebook/hhvm@cad8496353700ca846e32e9c8cfc4b20cd685e1c, which is in HHVM 3.2. The current alternative is `throw_not_implemented()`. We should probably have a macro for this purpose. It can check HHVM_API_VERSION and decide to either throw NotImplementedException or call the new function. I believe HHVM 3.1 has an HHVM_API_VERSION of 20131007L, and 3.2 uses 20140702L. Something like the following should suffice:

```
#if HHVM_API_VERSION < 20140702L
#define throw_not_implemented(msg) \
    throw NotImplementedException(msg);
#endif
```