Preserve support for Python 2.6+

[roskakori]

Goal: even after migration to Python 3 (#56), the code still works with Python 2.6+.

Notes on how to achieve this:

• each module starts with the following imports:

  from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from __future__ import unicode_literals

• use the six package for portable code, see https://pypi.python.org/pypi/six and http://pythonhosted.org/six/.
• fix all issues in the code pointed out with # TODO #61: ....
• decorate classes with an own __str__() with @python_2_unicode_compatible (currently found in _compat)

Instead of six and _compat we could also use future available from https://pypi.python.org/pypi/future.

[roskakori]

Note: I modified this ticket to cover all issues concerning the back port to Python 2 as this will be an ongoing topic for the weeks to come.

[roskakori]

Current status: all modules have from __future__ imports, 128 test pass and 28 fail when running tox -e py27.

Most of the failed test should be fixable by using six.text_type or considering the u'...' prefix for unicode strings when comparing expected error messages.

[roskakori]

All test pass with Python 2.6 and Python 2.7.