Python 3 Support

Added Python 3 support.  This required renaming unicode.py to unicode_chars.py (since the 2to3 process likes to rename all instances of 'unicode' with 'str') and a few changes to setup.py.  Also, the odict module will now only be listed as a requirement if using Python 2.6 or older (it was added to the standard library in Python 2.7).

This should correct upstream issue #23: https://github.com/rspivak/slimit/issues/23