Issue#60 - new db package with backend and iterator.

[samshub]

Usage:

db := db.NewMongoDB()
defer db.Close()

db.Find(...)
db.Upsert(...)

Hey @buth, I've merged your backend package with my code on mongodb. This is all in db package. Let's look at this together when you're ready.

[samshub]

Discussed the details with @buth and took care of few changes.