novice/git/img: Add viewBox to Computer.svg and GitHub images

Otherwise Firefox won't scale them [1].

[1]: https://bugs.launchpad.net/inkscape/+bug/168261
novice/git/img/github-collaboration.svg: Cleanup by hand

The Inkscape-generated content is long and hard to read.  The image
layout isn't that complicated, so just hand-code it.  I've left notes
where I calculated x and y positions, and used <image> tags to embed
references to Computer.svg, GitHub_Logo.svg, and OctoCat.svg [1].

[1]: http://www.w3.org/TR/SVG/struct.html#ImageElement
novice/git/02-collab.md: Use <object> to embed img/github-collaboration.svg

This is the preferred method for embedding SVGs, and is recommended by
Mozilla, who also say [1]:

  An img element can be used theoretically, too. However this
  technique doesn't work in Firefox before 4.0.

I'm running Firefox 24.8.0, and basic SVGs included via <img> do work,
but <image> calls inside those SVGs are ignored [2].

[1]: https://developer.mozilla.org/en-US/docs/Web/SVG/Tutorial/Getting_Started#Basic_properties_of_SVG_files
[2]: https://developer.mozilla.org/en-US/docs/Web/SVG/SVG_as_an_Image#Restrictions
Merge pull request #1 from wking/github-collaboration-svg-cleanups

SVG rendering cleanup for novice/git/img/github-collaboration.svg