add git submodule support

[mklooss]

added sub module support

[colinmollenhour]

I don't understand this PR.. If a repo has a submodule the --recursive option to the clone command will automatically init and update submodules... The "git submodule add" command will modify the .gitmodules file and cloning a module should not result in modified repo files..

[Silarn]

I believe the intent is to add the repo as a submodule to the parent automatically. I can empathize because I recently was setting up modman for the first time and when I tried to deploy to a new server, I discovered the repo had no actual data about the origins of the repositories, so I could not easily "re-clone" the modman files. It had to all be redone manually.

I personally have found it easier, in this setup, to simply use git to create submodules within the .modman folder. However, this leads to other potential problems if you then try to use modman to update those repos, one of which has to do with a change in git and how it handles submodule repo files. (No .git directory, just a .git file.)

I'll be creating another PR shortly that addresses this issue...