Session support

[antonkulaga]

I am not sure how exactly it should be implemented but I think that banana-rdf should have sessions, and methods like withSession{ con =>
//some code
}

[antonkulaga]

The idea to add a session emerged in a conversation with @inthenow at #274 where he told me that transactions should never close connections and I should better have a session for cases where I want to dispose of connection in the end.