Allow tasks to be signed and only execute signed jobs

[ripienaar]

Obviously we couldnt sign cron jobs but for one off this would be good.

The idea is that if the infrastructure runs in an untrusted location that we can control who can create tasks in it.

• Signing (#109) Sign and Verify tasks in the client and cli #111
• Loading (#109) Sign and Verify tasks in the client and cli #111
• container generator (#109) Allow verification keys to be used with docker containers #112