Add installer script

This pull request adds the installer script and instructions on how to use it for anyone trying to install popHealth.

Just a few notes:

First of all, the installer itself is able to run on any platform, however the chef scripts do fail on anything but Ubuntu. On fedora it fails due to the apt recipe trying to run and on mac I did not test it, but I assume it will fail for the exact same reason.

Second, the branch functionality will not work until this is merged into master, since in the installer the git clone script pulls from your master branch, which does not have the node[:popHealth][:branch] flag.