Make Bolts subproject referencable. Introduced build_release script.

This commit makes Bolts.xcodeproj to be subproject referencable in Xcode as well as introduces build_release.sh script to e2e build the releases of Bolts.framework.

- Got rid of Debug64, Release64 configurations
- Make arm64 compile with iOS 6.0+ deployment target on all configurations
- Got rid of custom build step to copy headers in favor of Xcode public headers folder path setting
- Added build_release.sh script
- Updated target configurations to be checked by Xcode 5.1.1

Test Plan:
- Used compiled Bolts.framework on iOS 5.0 Simulator on 10.7 via empty example app with some code that uses BFTask/BFAppLink
- Run `./scripts/build_framework.sh && otool -fV -arch all build/ios/Bolts.framework/Bolts` - make sure that you've got all required architectures in the project.
- Run `./scripts/build_release.sh`