action defined subcommand

  `juju action defined ...`
add patchAPIClient now that we need it
use jc.YAMLEquals
Merge pull request #1333 from johnweldon/actions-defined-pr

Actions defined

Implements 'defined' subcommand of `juju action`

Depends on upstream PR http://reviews.vapour.ws/r/616/

(Review request: http://reviews.vapour.ws/r/651/)