Make the 'spec:uncommitted' Rake task more useful

* Run all specs if there are files in 'spec' other than spec files that
  are uncommitted (this is useful for testing shared specs and fixtures)
* Run no specs instead of all of them if no files in 'spec' are
  uncommitted
* Notify the user if there are no specs to be run
* In the task description, show the number of uncommitted spec files to
  be run