add authorization guide + links to Google docs

[stephenplusplus]

fixes #483
fixes #479

Docs: http://stephenplusplus.github.io/gcloud-node/#/docs/master
Auth: http://stephenplusplus.github.io/gcloud-node/#/authorization

[ryanseys]

Only thing I notice is that example code on the Authorization page doesn't have a background around it, it's just on the white background which is different that the @example blocks in the other parts of the docs. Otherwise, it looks great :)

[stephenplusplus]

It turns out that implementing syntax highlighting is quite complicated. We're using a library to turn the Markdown into HTML, which spits out a plain <pre class="javascript"></pre> block. I did make a change to carry the base styles (background-color, padding, color) to the Markdown code blocks, however, which helps.

[ryanseys]

Sounds good.