[WIP] Record action execution end time, various improvements and bug fixes

Records the time when an action has finished on the `ActionExcution` model via the `end_timestamp` attribute.

It also includes bug-fixes for various bugs I've encountered while testing it and improves formatting for iso date time fields returned in the CLI:

Before:

```
+--------------------------+------------+--------------+-----------+-----------------------------+-----------------------------+
| id                       | action     | context.user | status    | start_timestamp             | end_timestamp               |
+--------------------------+------------+--------------+-----------+-----------------------------+-----------------------------+
| 5486103a0640fd2ca3c1eb70 | core.local | stanley      | succeeded | 2014-12-08T20:55:22.204000Z | 2014-12-08T20:55:25.792000Z |
| 548610540640fd2ca3c1eb71 | core.local | stanley      | succeeded | 2014-12-08T20:55:48.021000Z | 2014-12-08T20:56:29.401000Z |
+--------------------------+------------+--------------+-----------+-----------------------------+-----------------------------+
```

After:

```
+--------------------------+------------+--------------+-----------+------------------------------+------------------------------+
| id                       | action     | context.user | status    | start_timestamp              | end_timestamp                |
+--------------------------+------------+--------------+-----------+------------------------------+------------------------------+
| 5486103a0640fd2ca3c1eb70 | core.local | stanley      | succeeded | Mon, 08 Dec 2014 20:55:22    | Mon, 08 Dec 2014 20:55:25    |
|                          |            |              |           | UTC                          | UTC                          |
| 548610540640fd2ca3c1eb71 | core.local | stanley      | succeeded | Mon, 08 Dec 2014 20:55:48    | Mon, 08 Dec 2014 20:56:29    |
|                          |            |              |           | UTC                          | UTC                          |
+--------------------------+------------+--------------+-----------+------
````