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
How about something like:

> In a Run Script build phase, copies each Input File framework into the built app bundle.

The key points I want to hit home are:

 1. This is only meant for use from within Xcode
 1. The frameworks are specified as Input Files, not arguments