Expanded features section

So here's an expanded version of the features section: http://haskell-lang.org/

I've filled in content as best I can. After filling it out I thought it looked pretty good fully expanded, so I've included a commit which disables the expanding. I made it a separate commit so we can revert it later if we decide we need more space on the page (e.g. to fit events).

It'd be good to pass this onto one or two people unfamiliar with Haskell for a quick non-overcritical review and gauge their reactions.

Bear in mind: For each section I'd like to put a link at the bottom of the section to a full page which fully expounds on the idea. And a big prominent link to the /documentation page at the bottom of the page, so that you read all these sections, and if you're still interested you've got a clear way forward: straight to the books and such.