add Workflow.get_status()

Workflow.status() only logs status information. It will be useful to have a function that returns parsed pegasus-status output s.t. users can programmatically access it.

To preserve backwards compatibility, the idea here is to implement Workflow.get_status() (later we can implement Workflow.get_statistics(), Workflow.get_analysis(), etc) that will actually return a dict or object with the data rather than having Workflow.status() return that data, and thus making it unable to be chained.

Workflow.get_status() should return a dict containing:
- unready
- completed
- queued
- running
- fail
- state
- percent done
- raw output