add elemental

ping: @poulson

This adds a Linux-based Elemental build to hashdist, with the goal of eventually providing a simple, sturdy way of provisioning Elemental to desktop computers and supercomputers.

This PR also adds a simple "run-the-code" test.  @poulson, we can come back to this later and use assertions to provide more protection.  @poulson, you can also copy our .travis.yml file and remove the other tests for free Continuous Integration of Elemental on your repository.
This comes down to using environment variables vs. symbolic linking of packages in environments.  I'm not sure I have this right, but I'm also not convinced that I'm wrong here.