Add `carthage fetch` command

```
$ carthage fetch https://github.com/ReactiveCocoa/ReactiveCocoa.git
*** Fetching ReactiveCocoa
```

```
$ carthage fetch https://github.com/jspahrsummers/Notifications
*** Cloning Notifications
```

Resolves #116.
This refactoring was because I was originally interested in trying to use the `Project` type for `carthage fetch`. I decided it'd be a code smell after these bits were already changed, so that never happened, but I figure these changes are valuable and should be incorporated anyways.