Add use_path argument to Atlas.create()
defaults to False for consistency with the original behavior
When use_path is True the path is included in the id, with and slashes replaced by _
You can now generate an atlas with path info in the ids from the command line if you do
  python -m kivy.atlas use_path output size filenames
instead of the usual:
  python -m kivy.atlas output size filenames

(I would have loved to use the argparse module, but it conflicts with kivy's existing command-line argument handling)
Clarify documentation of use_path slightly
When creating atlas with use_path, strip leading dots and slashes
so that "../folder/filename.png" becomes "folder_filename" instead of ".._folder_filename"
Documentation fix to mention when use_path was added and a style fix
Merge pull request #1285 from bob-the-hamster/atlas-paths

Atlas paths