Adds more buttons to the top bar

We now have:
- Download
- Getting Started
- Resources
  - API Docs
  - InsightJS on Github
  - Report Issue
- Demos
  - ...

We use target="_new" to open any external links in a new tab, instead of
changing the window.
Add a "Fork me on Github" button to the page

Adds a button linking to the fork page of the repo, in light-grey at the
top-right.