docs(aliby): describe image module & improve class descriptions

WHY IS THIS CHANGE NEEDED?:
- an overview of what image instances do will help us understand and use
  such objects

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- add long docstring at beginning of module (similar to e.g. tiler)
- modify line in docstring for ImageDir.  it doesn't inherit from
  ImageLocal; it inherits from BaseLocalImage.

WHAT SIDE EFFECTS DOES THIS CHANGE HAVE?:
- need to check whether descriptions are accurate