Added Python 3 support using six.(for #91)

I tried to make pyjade compatible with Python 2 and Python 3 with the help of six.

This version passes the all testcases on CPython-2.6.6 and CPython-2.7.3.
On CPython-3.3.0, ('code', 'Mako') and ('code.iteration', 'Mako') always fails.
But this error seems caused by Mako's implementation.