Guide: intro edits

This is mostly organizational and simplifying. However, I also removed prose which I took to be too basic given that we expect this to be read by programmers. Comments welcome.

cc @steveklabnik 
You should probably link directly to reddit and IRC:
IRC: `http://chat.mibbit.com/?server=irc.mozilla.org&channel=%23rust`
reddit: `http://www.reddit.com/r/rust/`

IRC was taken from the rust home page.