Subtree command for git-tfs

I've written an implementation for issue #344.  
When executing the following command:
```
git-tfs subtree add -p=[prefix] [tfs-url] [repo-path]
```
git-tfs will:
* Locate or create an "owner" remote for the subtree, this remote has no TfsRepositoryPath but rather delegates to other remotes which represent its subtrees.  Uses "default" unless instructed otherwise.
* Create a subtree remote with ID `[owner]_subtree/[prefix]
* Set the subtree remote's workspace to be inside the owning remote's workspace, at `workspace/[prefix]`
* Fetch the subtree remote
* execute `git subtree add --prefix=[prefix] [subtree remote] -m [commit msg]`, where the commit message includes a git-tfs-id line for the subtree master.

Afterwards, the revision history will look like this:
```
commit e75165c22d0415613129cbc5456cc7b491ec6903
Merge: 845abef 5618bb9
Author: Gordon Burgett <gordon.burgett@gmail.com>
Date:   Wed Apr 10 13:38:32 2013 -0500

     Add 'InternalTools/' from commit '5618bb9065d9df8b059e7218db1a639e38a54f22'
    
    git-tfs-id: [http://my.server.url:8080/tfs/myco];C19373
    
    git-subtree-dir: InternalTools
    git-subtree-mainline: 845abef174122eb5f5985d899a3875be965654c7
    git-subtree-split: 5618bb9065d9df8b059e7218db1a639e38a54f22

commit 5618bb9065d9df8b059e7218db1a639e38a54f22
Author: Someone Else <someone.else@company.com>
Date:   Mon Apr 8 21:20:27 2013 +0000

    Fixes #1094 - some other task
    
    git-tfs-id: [http://my.server.url:8080/tfs/myco]$/Production/InternalTools/MAIN;C19370

```

From this point on, a `git-tfs pull` against the owning remote ("default") will pull all changesets across all known subtree remotes (in this case "default_subtree/InternalTools") and will apply the changesets.
A `git-tfs checkin` will pend all changes across all subtree remotes in the same workspace.
Similarly with a shelve or unshelve.

Things left to do:
* Modify Init-Branch to handle subtrees
* Add handling for pushing and pulling to a single subtree remote from within a subtree branch, for example after executing `git subtree split --prefix=InternalTools` I should be able to pull from "default_subtree/InternalTools" and checkin to the same.