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.
Should this method now be `readCombinedCartfile`, too?