Changed AFNetworking dependency to AFNetworking/NSURLSession.

Use CocoaPods subspec to only import what is needed.
Ignore Cocoapods stuff
Remove the Cocoapods files that shouldn't be in git.

These files shouldn't be built by the user.
Merge pull request #8 from otium/master

Add Cocoapods files to .gitignore since it shouldn't be checked into the main repository, and slim down the Podfile's AFNetworking dependency.