boxes and actions are stored in dictionaries
Merge branch 'master' into boxes-and-actions
hand-convert TA sample 'math-dice' to Python code
- adjust classes.py and output_example.py to turtle-centric TA, and
- replace DummyTAWindow with its parent class, TurtleArtWindow
  (depends on patch of TA)
adjusted sample graphics-tree.py to turtle-centric TA
adjust sample graphics-xo-child.py to turtle-centric TA
adjust sample math-dice.py to turtle-centric TA
Merge branch 'master' into boxes-and-actions; adjust to renamed module
Merge branch 'boxes-and-actions' into turtle-centric-2
add pretty-printing methods to `ast` module to ease inspection of ASTs
AST pretty-printer: replace str() by repr(); sort keys alphabetically
"hello world" example using codegen

hello_hand.py -- hand-crafted output
hello_gen.py -- code generation script
hello_out.py -- auto-generated output
"hello world" example using unparse
adjust to new distinction btw. running TA and exported code
Merge branch 'turtle-centric-2' into py-code-generation-ex