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.