Improve docstrings

[kyleabeauchamp]

For example, it might be nice to list the positional arguments on open():

In [6]: p = stor.Path("hey")

In [7]: p.open?
Signature: p.open(*args, **kwargs)
Docstring:
Opens a path and retains interface compatibility with
`SwiftPath` by popping the unused ``swift_upload_args`` keyword
argument.
Type:      method