(QE-613) Add `:catch_changes` to check for manifest resource changes

Many times when testing modules we apply puppet code twice in a row and
verify that the second run does not produce any resource changes to
verify that the resources are idempotent. The `:catch_changes` option
can be used to monitor for this result.
Merge pull request #94 from hunner/catch_changes

(QE-613) Add `:catch_changes` to check for manifest resource changes