Support building Calabash as a dylib

This pull request creates Xcode targets (and build scripts) for building Calabash server as a dylib for iOS.

Since Xcode doesn't support building iOS dylibs out-of-the-box, this does require changing the Xcode configuration. Ideally we will remove this requirement in the future.

Given the Xcode Spec files from [here](https://github.com/ddeville/Dynamic-iOS/tree/master/Xcode%20Specifications), I ran:

    sudo cp Xcode\ Specifications/iPhone\ Simulator\ P* /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/Library/Xcode/Specifications/

    sudo cp Xcode\ Specifications/iPhoneOSP* /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/Library/Xcode/Specifications/

With that in place, you can build the dylibs by running:

    make
    #...
    make dylibs
    #...
    # INFO: Copied dylibs to ./calabash.dylibs

and they will be copied to ./calabash.dylibs

(note to decrease size I've chosen to not use lipo to create a universal sim/device dylib).

# Important note
Installing Calabash server as a dylib will be considered an experimental feature for quite some time until we have proof of stability.