Integrate local non-git-repositories

It would be very nice, If you would allow to symlink to local projects without having git involved. The following `Cartfile`

```
local '/path/to/dependency'
```

could produce the following folder structure

```
Project
Carthage
Checkouts
local-dependency -(symlink)> /path/to/dependency
Build
local-dependency.framework
...
```

This would allow to work on a library and a dependent application at the same time without having to
- commit changes in the library repository and do a `carthage update` in the application folder (when linking locally via git)
- use submodules, which we encountered to be very prone to error when working with a larger team

This way, changes are applied directly when doing a build in the main application (when using automatic rebuilding) without having to worry about tangled git repositories. It would allow us to come very close to our current Cocoapods workflow but with reduced management overhead and an less intrusive way of integrating dependencies.