Port to Python 3

[Starou]

Use Future to make the codebase compatible with both Python 2.7 and Python 3.5+

• import open from builtins in all modules ? This will raise TypeError: write() argument 1 must be unicode, not str as a side-effect in components.py for example.
• In Python3, Should IDMLPackage.export_xml() return a string instead of a bytestring (from etree.tostring())?
• Scripts