Add more comments to fake in-memory filesystem
Add more comments to fake in-memory filesystem

Ensure users of this code/objects are aware that only
posixpath style paths will work (and not the variant
that is used in os.path which can change depending
on operating system) as well as adding docstrings on
other methods so that they show up in the generated
docs.

Change-Id: I5a21cb75b2452b9c9d65860b63658a9d0980025f

pombredanne/taskflow-1,junneyang/taskflow,openstack/taskflow,jimbobhickville/taskflow,junneyang/taskflow,pombredanne/taskflow-1,jimbobhickville/taskflow,openstack/taskflow