Major cleanup and update of the front page

The front page really needed a cleanup. We can argue over the specifics, but this should prettify it and bring it up to date and make it a lot more welcoming to someone new to ScalaJS =)

- Added uPickle and autowire to the list of libraries
- Added @sjrd's talk to the list of presentations
- Added the ray-tracer and papa-carlo-incremental-parser-demo to the list of sites using ScalaJS
- Converted list of libraries etc. from HTML lists to sub-headers, added SBT snippet for each one.
- Moved the 0.5.2 announcement to the bottom of the page, together with a list of other version announcements. For someone who just arrives at the site and wants to use ScalaJS, the version-release-announcement is probably of no interest
- Moved IMPORTANT NOTICE to the bottom of the page; I don't think it's *that* important that we should shove it in the face of everyone who comes to look at the site