Add script phase to generate build/BCOVRAC.h

This is the file whose preprocessor directives are used to add the `BCOV_`
prefix to all ReactiveCocoa symbols in BCOVPlayerSDK. This commit depends on
Tools/Generate_BCOVRAC.sh, which will be added to that repository.
Rename target to `Generate_BCOVRAC`

Previously it was `Copy_libRAC`, but since it now does that only for the sake
of generating BCOVRAC.h, this new name is more appropriate.