Don't link RAC into iOS library target

Consumers may have multiple copies of RAC, so they should link their
desired copy at the application level.
Explicitly build Archimedes and RAC before RCL

This ensures that their headers are copied into place before trying to
compile RCL.
Merge pull request #103 from ReactiveCocoa/fix-ios-linking

Don't link RAC and Archimedes into iOS library target