Rewrite and reorder README.md's beginning

Rewrite the summary description of Meteorite.

Add a quick demonstration of Meteorite's capabilities and interface, underneath
the summary. This is a common (modern) convention of utilities' READMEs.

Rewrite the installation instructions to include links to `npm`, to
help out newbies to the Node.js world.

Replace the commands for fixing Ubuntu 12.04 installations with a link to the
PPA. Ubuntu users should know how to install PPAs, but if not, the Meteorite
README probably isn't the best place to learn. There's no need to bother
non-Ubuntu users with those instructions. Also, it's not the only way to
install a recent version of Node, and so it shouldn't be presented as such.

Move the installation instructions to the top of the README, to be more visible
to readers. Utilities like Meteorite almost invariably have installation
instructions close to the top.