Also store time when the action has completed on the action execution model.
Display execution end timestamp in the cli.
Update affected fixture.
Update affected tests, add new test case.
Add support for formatting instance values and make start_timestamp and
end_timestamp attributes human friendly when listing and getting action
executions.
Fix JSON cli formatter so it works correctly for other list like objects
(tuples, iterators).

Previously, JSON output didn't work for "execution list" because
listreverseiterator != list.
st2client now also requires python-dateutil.
Update affected fixtures.
Return an error if user doesn't provide a body with the POST request which
require it.

Previously, internal server error was thrown.
Handle requests which require body better and remove now unnecessary
requires_body argument.
Update changelog.
Merge pull request #880 from StackStorm/record_end_time_in_action_execution

Record action execution end time, various improvements and bug fixes