Add build target in UIKit project to build SpecHelper framework

The part of PCK I use most frequently on projects is the UIKit SpecHelper classes.

To make adding these classes to a project easier, I added a build target in the UIKit project and a rake task to build a static framework that includes all of the PCK UIKit+SpecHelper header files.  To give it a spin, go to PCK root and type:

`rake uikit:build:spec_helper_framework`

it will build the universal static framework into:

`build/Release-iphoneuniversal/UIKit+PivotalSpecHelper-iOS.framework`