Added channel-based synch introduced in pr #20.
sched.Start(), sched.Join(), sched.Run() run/tested OK.
Added LaunchTasks(), killTasks()
scheduler registration working, better tests, additional http-headers, and http response validation.
Move executor example under the examples dir.
Added handler for frameworkreregistered message.
Added scheduler driver resource offers handler.
added resource offer rescinded handler.
moved schedulerdriver interface to schedtype.go