* Allow the Trogdor agent to be started in "exec mode", where it simply
runs a single task and exits after it is complete.

* For AgentClient and CoordinatorClient, allow the user to pass the path
to a file containing JSON, instead of specifying the JSON object in the
command-line text itself.  This means that we can get rid of the bash
scripts whose only function was to load task specs into a bash string
and run a Trogdor command.

* Print dates and times in a human-readable way, rather than as numbers
of milliseconds.

* When listing tasks or workers, output human-readable tables of
information.

* Allow the user to filter on task ID name, task ID pattern, or task
state.

* Support a --json flag to provide raw JSON output if desired.

Reviewed-by: David Arthur &lt;mumrah@gmail.com&gt;, Stanislav Kozlovski <stanislav_kozlovski@outlook.com>