Leverages recent changes in diff-cover to reduce number of diff-quality runs

TE-555

This adds the -l switch, wherein a violations limit is passed in. For
example:
  paver run_pep8 -l 700

Will fail if more than 700 violations are found.

By default, this limit is not enforced (i.e. if the -l switch
is not passed into the command, then a limit will not be put
into effect for that run)