MongoDB storage driver with unit/functional tests

This commit builds upon @iampersistent's work in bernardphp/bernard#94.

The driver is now constructed with MongoCollection objects, which removes the previous MongoDB::selectCollection() logic. Additionally, the message sentAt field is now a BSON date and queue name was added to acknowledgeMessage()'s remove() criteria to be consistent with the SQL driver.
Do not require MongoDB extension, use mongofill on HHVM
Rewrite MongoDB driver documentation