Use Illuminate/Database/ConnectionResolverInterface in LMongo

Currently, LMongo bundle implements its own ConnectionResolverInterface which is very similar to the standard Laravel's resolver defined under Illuminate/Database/ConnectionResolverInterface.

Switching to the standard interface will allow better reuse of the Illuminate's code. One example is that if we make LMongo/DatabaseManager implement the standard interface instead of LMongo's resolver interface, we will be able to inject LMongo/DatabaseManager anywhere Illuminate/Database/DatabaseManager is expected. That, I believe, will open some interesting opportunities for easy switch from RDBMS to Mongo database

This is a very simple change. Please, consider pulling it.