Feature: Health checks

@ssorallen Would you take a look at the taskDetailsComponent update?

I updated this page https://github.com/mesosphere/marathon/wiki/Test-App-Commands to contain the toggle server i build to be able to toggle the health of the end point of the health check.
Could this get a `title` attribute so hovering on the `<td>` revealed a tooltip with the message? If full failure message is appropriate, that works. Whatever you think is appropriate.