Various improvements to the smoke tester

* Fix the wait_for_workers in 'jobs' mode
* Handle a dep dir being a symlink (leftover from an aborted previous run)
* Add a new --tasks option so you can pick which tasks you want to run (eunit/dialyzer/xref)
Merge pull request #532 from basho/adt-smoke-test-cleanups

Various improvements to the smoke tester