Rake tasks

[mhyee]

We're finding that it's a lot easier to do stuff via the console, as opposed to the web interface. There are a lot of common tasks we run, which should be coded up as Rake tasks.

For example (incomplete list, feel free to add more):

• Clean up all dead workers, delete the orphaned test results, and requeue tests
• Clean up all dead workers, delete the orphaned test results, but don't requeue tests
• Run all {Continuous Integration, Correctness, Performance} tests (but exclude tests that have never completed) on a specific commit
• Run a {Continuous Integration, Correctness, Performance} test on a specific model (or list of models) on a specific commit
• Distinguish between tests on our "prioritized" models (case studies on multiobjective problems) and everything else
• Delete all orphaned test results (e.g. tests that have started but do not have associated workers)

This issue supersedes #30