Doc: Improve README for AWS and CWL running
Doc: Improve README for AWS and CWL running

Describe symlinking CWL based tools for running bcbio and better
define when you need to install a full bcbio-vm installation with tools
and data versus only the python wrapper. Fixes #140

chapmanb/bcbio-nextgen-vm,chapmanb/bcbio-nextgen-vm