Modified pymongo/connection to allow mocking the connection with MongoMo...

Modified pymongo/connection to allow mocking the connection with MongoMock and similar libraries.

This is an experiment, if you think that this is not the correct way then I'm open to better proposals. My aim is to decouple a little bit MongoEngine from pyMongo in order to allow testing without network connection or the need to install Mongo in our machines.

This pull request is not enough to achieve my goal. I've pull requested another little change to MongoMock.

Thanks for your attention.

Code Example:

    from mongoengine import connect, ConnectionError, Document
    from mongoengine.fields import *
    
    from mongomock import Connection as ConnectionMock
    
    
    connect('UserDB', 'UserDB', mock_db = ConnectionMock)
    
    class User(Document):
    	meta = {
    		"db_alias": "UserDB"
    	}
    	email = StringField(required=True)
    	first_name = StringField(max_length=50)
    	last_name = StringField(max_length=50)
    
    ross = User(email='ross@example.com', first_name='Ross', last_name='Lawley').save()
    neil = User(email='neil@example.com', first_name='Neil', last_name='Armstrong').save()
    
    for user in User.objects:
    	print user.first_name