Provide for `subliminal-test` to use the Xcode 4.6 toolchain
while 5 support is pending. (fixes #33, refs #32)

This entails setting the Xcode directory to that of Xcode 4.6 before launching
`subliminal-test`, as described in the documentation, as well as forcing the CI scripts
to open the Simulator corresponding to the currently selected dev toolchain: the
Xcode directory in-and-of-itself has no effect on which "iPhone Simulator" is launched.
`subliminal-test` now properly builds a workspace if one is specified. (fixes #31)