Allow multiple owners to be contacted if job fails.

Allow Chronos users to input a comma separated list of owners when creating a job. Mail gets sent to each address in the list when a job fails or exceeds the configured number of retries.

NB: I'm aware the implementation of this is slightly kludgy. I had a nicer solution in mind where we change the model of a Job such that the "owner" attribute (currently a string) becomes a Set[String], but this solution required changing some JSON Serialization / Deserialization logic, and ultimately it seems like simply doing the string splitting right when we want to send mail is a functional and simple solution. I'd like to hear feedback on whether or not you'd prefer the more elegant solution (I've got some changes stashed that should be easy to get at)

This change should probably be documented in the readme if we decide to merge. 

@hcai @brndnmtthws @andykram