Remove the use of session key

Removed the use of the session key and replaced it with Connection. This solves issue #23.

P.S. I was not able to run the tests as I have no assets used by the client on my machine:

http://localhost/models/megacity/megacity.xml
http://localhost/models/natalieFives/xml3d/natalie.xml

I believe they must not be published in the open (i.e. GitHub), but may be used in screenshots etc. Please let me know from which internal repository can I get them, so that I can install them locally and run the tests.