Add :teardown-on-success

Allow teardown of nodes only when a test succeeds.
Add debug output to lein plugin
Add with-group-spec macro to cut test boilerplate

The with-group-spec macro wraps a test that requires the specified
group-spec. Node creation is put in a `startup` block and node teardown
in a `teardown` block.  Adds exception handling to ensure teardown
occurs correctly.
Updated project.clj, release notes and readme for 0.1.5
Updated version for next release cycle