Swap out session configuration for URLSession

[designatednerd]

The code changes here are pretty minimal: Swapping out a URLSessionConfiguration for the actual URLSession. However, by being able to pass in a URLSession that you control the delegate of, you can more easily support a number of things:

• Certificate pinning
• Self-signed certificates
• Authentication challenge responses
• Metrics inspection

Note that this is going to be a breaking change if you were previously passing in a session configuration to the HTTPNetworkTransport - you'll need to hand that configuration to a URLSession yourself first.

[designatednerd]

Blergh, something is up with Travis - it's refusing to install the starwars-server locally, so all the tests dependent on local are 💣ing out. I'll have to get a look at this late tonight or tomorrow.

[designatednerd]

Woot, clearing the cache did the trick. Merging!