Add :force_rebuild option for vendored xcodeproj

This PR adds a `:force_rebuild` option to vendored Xcode projects, which will attempt to rebuild the Xcode project every time.

My use case is that I'm using RubyMotion to test my existing Objective-C project, so the vendored project will change quite often. If no code has changed, only the `libtool` step runs so it's quite fast.