Clean up handout - Githubissues

I've had a go at cleaning up the handouts and I've made a number of changes:

Amalgamate the two handouts into one (which is now two-sided)
Add some new entries for commands and terms which are used in the course but weren't included
Drop panel on "Why to use VCS"
Fix up broken URLs and make them into clickable links
Make a separate makefile for the handout for readability

I figured that it made more sense to provide this material as a cheatsheet rather than a class handout. I don't think we'll be providing printouts (right, @AdrianDAlessandro?) but we'll link to the PDF in the course materials (see #70) -- hence why I think it's better to have clickable links.

There's a bit of a gap at the bottom of the first page -- any ideas about formatting etc. are v welcome!

For convenience, here's what the new PDF looks like:

handout.pdf

@cc-a I'm tagging you in this too, because I guess we might want to do something similar for the intro course?

Closes #16.

ImperialCollegeLondon / intermediate_grad_school_git_course

Clean up handout #71