- Adds a utility method to delete partitions older
than a specified threshold. This method wraps an existing
refinery script used to purge data.

- Adds a decorator to execute Python scripts by wrapping
the `BashOperator`. This approach is designed to facilitate
swapping in the `SkeinOperator` once the refinery Python code
is extracted into a standalone module.

- Mocks `os.path.isfile` in the `dagbag` test fixture so
that it always returns `True` when tests involving `dagbag`
include tasks requiring access to 
`/srv/deployment/analytics/refinery/bin/refinery-drop-older-than`.
Without this mock, refinery must be deployed on the test infrastructure. As noted in the comments, this is a workaround necessitated by the fact that refinery Python modules are not yet packaged and bundled.