Modify KIF to be XCTest and SenTestingKit compatible

Using preprocessor directives, I modified the files that were specifically referencing SenTestingKit to use XCTest if XCTest is imported otherwise default to keeping SenTestingKit.  I created an additional static library, KIF-XCTest, that users can import into their projects if they want to use the XCTest version.  Also, the podspec now uses subspecs so users can pick if they want to use the XCTest compatible version by specifying `pod 'KIF/XCTest'` or the SenTestingKit one, `pod 'KIF'`.  The podspec defaults to the SenTestingKit version so people already using KIF won't have their tests broken.