Added expires field to job definition

This will be used so jobs created during integration tests
can be garbage collected by the master. It can be passed to
the CLI as an ISO-8601 string.
JobTest now tests the expires field
ExpiredJobReaper: initial implementation

Periodically poll for expired jobs, and undeploy and remove them.

This should eventually be moved out of the master into its own process.
Merge pull request #52 from spotify/rohan/deadline

Implement job expiration and reaping