py3k compatibility

[dx]

We need to make all the changes possible to make the transition to py3k really smooth.

• Change all the print to debug() calls. debug() should be a global function in a "global" module
• % operator is gone. We'll have to implement a format() function for py2.x, or use the 'str' + str(123) + 'str' ugly syntax everywhere.
• (quote) "String exceptions are gone (of course)."
• (quote) "The except E, e: syntax changes to except E as e". We should avoid {{{except E, e}} where we can
• (quote) "The old raise syntax variants raise E, e and raise E, e, tb are gone."

That's everything I think that affects us. Check http://www.artima.com/weblogs/viewpost.jsp?thread=208549 for all the other changes (well, just highlights)