Introduce Unit Testing & CI Setup for AFNetworking

Per the discussion of #941, I have taken an initial cut at setting up the testing environment. There are a few items worth mentioning:

1. I had to add an AFNetworking.xcodeproj so that the tests have a place to live. In doing so, I introduced a static library and a framework target as well as a pair of logic test targets. I know @mattt has been resistant to having a static library target in the past. I believe we could get away with collapsing the project down to only containing the Test targets if desirable, but thought it would be worth a discussion since we now need to keep an xcodeproj on tree.
1. I threw together a lightweight Rake task for running the tests on CLI and via Travis CI. In order to do this, you need to carry around shared Xcode schemes as xctool cannot detect the implicit dependencies between the test schemes and CocoaPods. I just setup Xcode to hide them in the Scheme editor, but they will appear in your menu on each fresh checkout of the code base.

I only have a few tests in the harness at the moment -- wanted to get this on the table for consideration and then we can all put our heads together around what the most valuable coverage is.