Add a target to the iOS Xcode project to build a universal (arm + simulator) static library for inclusion in other projects

Per some comments on the website, it can be useful to build MKNetworkKit as a separate static library. On iOS, this is best done as a universal library that has both arm and x86 code for use on iOS devices and the Simulator. However, Xcode makes this a stupidly difficult task. This patch adds a new target to the project file to automate the process.

The method here is similar to the Facebook iOS build script and that described in http://blog.boreal-kiss.net/2011/03/15/how-to-create-universal-static-libraries-on-xcode-4/. To prevent misunderstandings and confusion, the static library target always builds a release build of the library.