Rewrite ModernHttpClient.iOS to use NSUrlSession

This PR removes AFNetworking and replaces it with iOS 7.x's NSURLSession. This both fixes the race conditions people got with AFNetworking, as well as makes the iOS library 1MB lighter, at the cost of not being iOS 6.x compatible.

Fixes #22, most definitely.

/cc @gshackles, @JaccoRademaker, @jdehlin, @nberardi, @ChuckPinkert, @Iain1986 - can you guys please try this version and see if it fixes your issues? You can find an updated version here, it should be a drop-in replacement: http://cl.ly/212Y0m1T3r43