Add "Kill" and "Kill & Scale" buttons for tasks

Kill & Scale is currently only available for one task at a time due to
a bug in the Marathon client. By allowing only one at a time, it
throttles the requests and ensures the scaling happens properly.

Make table rows look clickable with pointer cursor
Merge pull request #113 from mesosphere/kill-task-modal

Add "Kill" and "Kill & Scale" buttons for tasks