Add `copy-framework`

As mentioned in #188, this command can be used to

1) Copy a framework into an app bundle
2) Strip all unwanted architectures (think `x86_64` slices when building for ARM)
3) Re-sign the framework as needed

This changeset is now functional.

What's left to do:

- [x] Read architectures in the binary from `lipo`
- [x] Actually copy the frameworks that are passed in
- [x] Tests
If it's `nil`, its `code` won't equal `NSFileNoSuchFileError`, so I'll default to a `RACError.Empty.error` in the next line, too.