Use defaults if per-project overrides aren't set in chef-rundeck.json.

Tests for an issue uncovered by #53.  This should resolve that
particular issue but is not an ideal implementation.  It's not too
late to fix it!  Fix it good!
Basic data validation on username parameter to fix #53.

Again.  Not the best possible fix but it moves us in the right
direction.
Adding the solutions to some GitHub questions to the README.

Specifically fixes #39, may head off duplicates of some others.