Change "git-core" to "git"

Pull requests #1 and #2 change the name "git-core" to "git" for Debian
and RedHat systems, respectively. It looks like "git-core" is deprecated
or on its way to deprecation everywhere so let's just make it official
and update the Modulefile version along the way.

Fixes #1
Fixes #2
Update README file

Include reference to the platforms known to work with the module
Merge pull request #3 from reidmv/git_core_is_deprecated

Change "git-core" to "git"
Rename README to README.md

Adding the markdown extension so that Github knows how to properly
display it on the website.
Merge pull request #4 from reidmv/markdown_readme

Rename README to README.md
Update copyright information.
Merge pull request #6 from apenney/LICENSE

Update copyright information.
Add git::subtree class to install git-subtree
Add some unit tests
Adding classes/facts to README
Merge pull request #1 from hunner/subtree

Adding classes/facts to README
Merge pull request #7 from mcanevet/subtree

Add git::subtree class to install git-subtree