Add job dependencies

[bschroeter]

Following from #10

It would be great to be able to specify job dependencies for a submission.

https://opus.nci.org.au/display/Help/Job+Dependencies

For example:

# Submit the first job
job1_id = client.submit("job1.sh")

# Submit another job that requires the first to complete
job2_id = client.submit("job2.sh", depends=job1_id)

There would need to be different arguments to specify the behaviour as noted in the link, such as running in the case of a failure or success of another job etc.

[bschroeter]

This may also be highly SCHEDULER DEPENDANT, so not all scheduling systems will have such facilities.

This issue will focus on PBS.