Reorganize Git install, setup, test-drive and daily-usage instruction.

See [rendered version](http://bernhardkonrad.github.io/STAT545-UBC.github.io/).

Check the new [hub for all git-related install, set-up, and workflow](http://bernhardkonrad.github.io/STAT545-UBC.github.io/github.html).

This is a draft, please contact @BernhardKonrad before merging. At the very least I'm sure @jennybc wants to suggest a naming scheme for the new files.