docs: fix examples, style optionals

The live site has a couple issues I didn't catch during PR review time (:():

- The sidebar active link styling isn't working
- Some examples are cut off (ex: http://googlecloudplatform.github.io/gcloud-node/#/docs/v0.6.0/datastore/dataset)
- We had some old code examples (`new gcloud.storage.Bucket()` etc)

I added some tweaks that hopefully add some readability:

- Styled optional parameters minimally (italic + `optional` prefix)
- Moved `> Getting started with gcloud` block to the top of the page

Waiting on my gh-pages to build, then I'll have a live example up. Keep an eye: http://stephenplusplus.github.io/gcloud-node/#/docs