Drop support for Python 2.5 for Travis CI

Fixes issue #21. Travis CI has dropped support for Python 2.5, thus we can do
so as well.
Move python3 specific tests to test/py3

These tests are now included in the source distribution.
Make dependency on nose for tests explicit

In setup.py the test suite is defined as nose.collector. This requires
the nose module to be installed.
Create unit tests from the existing doctests
Fix test problem for old style classes
Merge pull request #24 from coldfix/cleanup-tests

Cleanup tests
Merge pull request #22 from aepsil0n/fix/travis_drop_py25

Drop support for Python 2.5 for Travis CI