ENH: new subcommand `batchit interactive`

[brentp]

this has all the arguments that submit uses, but lands the user in an interactive prompt.
will also require --pem to get into the instance. this will be quite useful for debugging.

the actual impl can just submit a job that sleeps. so additional args are:

   --time INT        time in minutes for the job to last [default: 120
   --pem  PATH   path to .pem to be used to connect to instance

[brentp]

see: http://blog.ralch.com/tutorial/golang-ssh-connection/