Added support for JobNotification - INVALID, STARTED, SUCCESS and ERROR ...

I added job notification support based on our conversation here: https://groups.google.com/forum/#!topic/spark-jobserver/ZPrSWRcUgq4 . The /jobs endpoint now takes an optional callbackUrl parameter and makes a GET call to the url, along with jobId and status information. The status information can be INVALID, STARTED, SUCCESS and ERROR. 

I had to modify, StartJob, JobInfo and tests related to them -added a new Actor for notifications using spray http client. The Dao's had to be modified to store the optional callbackUrl. I did not create any new tables.  
I have tried to follow the patterns in the master -let me know if something needs to be improved. I have tested the server with a standalone spark cluster. 

I think you mean `callbackUrl: Option[String]` right?