Accurate Debian instructions

The instructions now match reality.  The EC2 instructions should
entirely copy the /install instructions (what's the node equivalent
of a Rails partial?), but for now there's duplication with a link
to the canonical page.

I made some minor English tweaks as well.

When this is merged and deployed I'll make the debian.neo4j.org instructions reference /install.  Should we also redirect debian.neo4j.org to www.neo4j.org/debian?