title: Include github packages as imports in DESCRIPTION
body: <p>I'm using devtools and I have some packages on github.  I would like to create dependencies between them, so when I run <code>install_github(...)</code> the other github packages that are in the DESCRIPTION file listed as Imports will also be installed.  Can I do this or is there another thing people do?</p>
  
  <p>Currently if I add a package to Imports that isnt available on CRAN I simply get a message "Skipping ... packages not available: xxx" when I run <code>install_github</code>.</p>