Clean up install script

[ebuchman]

Somewhat related to #2

I found the script a bit weird, and ended up copying and pasting what I needed.

For instance, it installs git. I think it's safe to assume the user already has git.

It also seems to assume the user doesn't have the local repo. I think we can expect folks to clone the repo and build from within the repo. This should simplify all the /tmp stuff.

Folks likely already have some version of node, and maybe don't want to use brew for it.

I think it would be best to include instructions for installation directly in the readme, consisting of a very small set of commands necessary to install the binary, assuming the dependencies are already installed and we're inside the repo.

Oh, we should also mention that we expect $EDITOR to be set.

[ebuchman]

:doh: it occurs to me now that what I was looking for was directly below, under Other Systems. This was a bit misleading :)

[zramsay]

looks like this was for the old version. now we have brew and and working makefile