Networkless checkout/update

It'd be awesome to be able to use `carthage` while not having internet access.

We'd need to:
1. [Support fetching refs](https://github.com/Carthage/Carthage/issues/116) ahead of time, so they can be checked out later without a connection
2. For `carthage update`, specify that we don't care about retrieving the latest list of tags (just use whatever was fetched already)
3. Somehow support initializing/updating dependencies' submodules without an internet connection

The final piece is really the hardest. Maybe we could keep bare clones of dependencies' submodules too?