Added start_request for MongoClient & Use kwargs for support all find options

For support [start_request](http://api.mongodb.org/python/current/api/pymongo/mongo_client.html#pymongo.mongo_client.MongoClient.start_request) and [all find options](http://api.mongodb.org/python/current/api/pymongo/collection.html#pymongo.collection.Collection.find)
I'm strongly against `**kwargs` here. Implicitly allowing kwargs without treating them properly might:

1. allow typos in tests go unnoticed
2. ignore requested features, thus returning wrong results, and once again differentiating between tests and real life scenarios

I rather unknown kwargs to explicitly fail, allowing users to skip tests selectively, or have mongomock properly support them, even in a naive way...