Add dedicated targets

This (in part) resolves issue #841

It has been tested for functionality with multiple user targets and subspecs.  The primary goal was for complete spec source isolation which was mostly solved by adding the per-spec targets as well as adding the USE_HEADERMAP = NO option to the Pod targets.  This is not added to the xcconfig file.  It's possible that setting USE_HEADERMAP = NO will break other libraries that do not do properly include headers by specifying the correct paths.