XD-1120 Standardize Date/Time/TimeZone handling

* Add custom `ISO8601DateFormat` class for Jackson that supports milliseconds
* Configure REST support to serialize dates using ISO8601
* Add class `TimeUtils` to centralize any `DateFormats` or retrieval of `TimeZones`
* Support UTC as much as possible
* Polish the Shell and the Admin UI
* Add `moment.js` so that ISO8601 dates can be properly handled in the Admin UI
* Add AngularJS directive to centralize the rendering of *durations*
* Fix several tests

Jira: https://jira.spring.io/browse/XD-1120
@ilayaperumalg How about `getClientTimeZone`? Since users may need to understand the dualism of of the timezone concept within Spring XD (client time vs. server time). `configuredTimeZone` might be too ambivalent.