Change Carthage paths

This will be the new directory structure:

```
Cartfile
Cartfile.resolved
Carthage/
    Build/
    Checkouts/
```

Resolves #107 and #132.

**To do:**

 - [x] Scrub documentation for `Carthage.build` and `Carthage.checkout` references
 - [x] Ensure that all creation of build/checkout folders correctly creates intermediate directories
 - [x] Rename `CartfileLock` type
 - [x] Automatically move `Cartfile.lock` to `Cartfile.resolved`
 - [x] Automatically move `Carthage.build/` and `Carthage.checkout/` to their new locations

/cc @tonyarnold @raven @adamgit @dannygreg