Tweak README explanation about execfile()
Add paired test scripts for futurize

* For each script, the original Python 2-only version is written as
* myscript.py2, while our desired output of futurize is myscript.py.
Add another pair of test scripts for futurize
Start a todo list
Update new open() function docs (and fix bug)
Add some more useful Py3 / porting links
Add open() function from io module; improve the docs
Replace various internal imports with ``from future.builtins import *`` instead of ``from future import *``.
Small tweak to docs