Remove explicit support for Node 0.6.x

We have been having issues when running Travis builds on Node 0.6. None of these issues have been seen with 0.8 or 0.9. Node 0.8 has been out since last June so we're going to go ahead and remove support. I've linked to Harry's tweet confirming that we can remove support. As much as I would like to know why it's happening I'm going to move on as there is way more important work to be done.

https://twitter.com/harrybrundage/status/303272622647549953

- Remove explicit support for Node 0.6.x
- Update and render the docs
- Update the introduction to the documentation to properly state that we support node 0.6.x

@nickjs Please review

/cc @hornairs @cjoudrey