Describe how it returns a promise.

Closes #1
Don’t delete entire lib dir
Just use process.arch

I’m sure there are problems related to this with Windows, but I’m just
focusing on OS X for now.
Download node if one isn’t available

Because machines running atom-shell may not have node.js installed, this
insures that a node.js binary will be installed for npm.
Ignore ./bin
Make sure ./bin is created
0.8.0