update to mongo driver 2.2.3

[paolosanchi]

I've done the same job has sharnol did..

• updated mongo to v2.2.3
• fw update to 4.5 (minimum required by the driver)
• update to the tests
• mantained backward compatibility to the legacy driver (and tests as well)

Here an explaination to the last point (as you can also see in the tests, too): With the new driver you should use [BsonId(IdGenerator = typeof(Int32IdGenerator<StubInt32Entity>))] With the legacy driver you should use [BsonId(IdGenerator = typeof(Int32IdGenerator))]