Action supercommand

Use JUJU_DEV_FEATURE_FLAGS envar with an option for "action" to enable
this supercommand.

For example:

   export JUJU_DEV_FEATURE_FLAGS="${JUJU_DEV_FEATURE_FLAGS}, action"
address feedback on review

 - Test with and without feature flag
 - fixes for feedback trivials
 - don't use custom struct for action supercommand
 - use anonymous struct to serialize JSON
address review from ericsnow
don't include un-implemented commands
address more issues from the PR
add another TODO comment from review
Use IsolationSuite, and remove unused func (for now)
Fixes LP-1403596

  OS X (darwin) uses stat.Ctimespec not stat.Ctim

  updated per review comments
Merge pull request #1337 from johnweldon/lp-1403596

Fixes LP-1403596

  OS X (darwin) uses `stat.Ctimespec` not `stat.Ctim`

(Review request: http://reviews.vapour.ws/r/655/)
Merge pull request #1302 from juju-actions/cli-action-supercommand-pr

Actions :: CLI supercommand :: with feature flag

Initial supercommand for `juju action` 
Currently behind a feature flag so the `JUJU_DEV_FEATURE_FLAGS` envar should include a feature `action` to enable this supercommand.

(Review request: http://reviews.vapour.ws/r/616/)
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/)
state: automatic multi-env handling for DB updates

Transaction operations that update collections that contain documents
for multiple environments are now automatically updated to include the
required environment specifiers.
Merge pull request #1291 from mjs/auto-env-uuid-for-txns

state: automatic multi-env handling for DB updates

Transaction operations that update collections that contain documents for multiple environments are now automatically updated to include the required environment specifiers.

(Review request: http://reviews.vapour.ws/r/605/)
Add capability to cache lxc images used for starting lxc containers
Rework image storage implementation, address some of the review comments
Refactor lxc container manager to use an imageURLGetter
Add support for wget to use --ca-certificate to get images