Change code highlighting delimiters to use github-style code fences

[durden]

We currently use a format like this to do syntax highlighting the markdown text:

[code lang="python"] def x(): return 1 [/code]

However, it might be more obvious and convenient if we switch this to use the code fencing style that github itself uses:

def x():
    return 1

The reasoning for this is just that we are already tied pretty heavily to github so using their syntax seems like a bonus.

[glenbot]

Added github style code fencing fa1c4b6