Refresh README file a bit on first readthrough.

Cleans up some formatting and wording
Clarify the overview a bit, including Salt and Vagrantfile
Server base should be a second level heading
Clarify local development portions of the README

Working toward documentation that supports both GUI and CLI users of Git.
Update wording around Git installation
Trim trailing spaces in README
Merge pull request #5 from washingtonstateuniversity/readme-refresh

Refresh README