MongoClient::close() argument is optional
Use throw_not_implemented() instead of NotImplementedException in HHVM 3.2+

Fixes #60. See: https://github.com/facebook/hhvm/commit/cad8496353700ca846e32e9c8cfc4b20cd685e1c
Merge pull request #64 from 10gen-labs/fix-not-implemented

Fix NotImplementedException incompatibility for HHVM 3.2+