Changes Nitrous signup directions

Fixes #32
Adding custom css file link
Adding custom css file

Fixing heading sizes
Fix font size in CSS

Fixes #33
Add Git/GitHub explanation

Try to give a basic intro to what Git and GitHub are.
nav direction change

Fixes #34
Adds training explanation

Fixes #21
Added explanation about imagemagick

Nitrous comes with this, so students don’t need to do it.