Modify KIF to be XCTest and SenTestingKit compatible via cocoapod subspecs

I modified the files that were specifically referencing SenTestingKit using preprocessor directives to use XCTest if XCTest was imported otherwise default to keeping SenTestingKit.  I created an additional static library and test target so the tests can run on XCTest and SenTestingKit for the demo code.  Also, the podspec file now uses subspecs so users can pick if they want to use the XCTest compatible version or the SenTestingKit one.  The podspec defaults to the SenTestingKit version so people already using KIF won't have their tests broken.