Option to Shutdown when Render Completes

Feature implementation for llbit/chunky#24

* Added checkbox on Advanced tab of Render controls labeled "Shutdown when render completes" 
  * Toggleable at any time during rendering
  * State not retained at all (defaults to disabled, unset when Render controls are closed)
* Cancelable 30 second countdown before shutdown
  * Windows adds another 1 minute cancellation window (non-reconfigurable, AFAIK)
  * Unix-like shuts down immediately
* Amended README to enable functionality on Unix-like systems (Linux, BSD, OS X)
  * Requires use of `sudo` without password (so it doesn't need to prompt the user for it), instructs user to add `NOPASSWD:` directive for their username and `/sbin/shutdown` with `visudo`
* Actual shutdown command can be disabled for testing purposes by updating `boolean` constant [`ShutdownAlert.Shutdown.DO_SHUTDOWN`][1]. When disabled, prints the shutdown command for the host system to debug log instead of running it.

Tested and working on Debian Wheezy and Windows 7, both 64-bit with 64-bit Java.

Copied SystemUtils and JavaVersion classes, retaining attribution and license, from Apache Commons-Lang3 to facilitate robust detection of OS. JavaVersion was required to fix import errors in SystemUtils.

Merged with latest commit of llbit/chunky

[1]: https://github.com/cybergeek94/chunky/blob/master/chunky/src/java/se/llbit/chunky/renderer/ui/ShutdownAlert.java#L64