add unit tests and fix exception clause compatibility

Exceptions are now compatible with both python 2.4 and 3.3,
and we have unit tests to detect regressions.

The syntax for capturing exceptions changed in python 3, but
we need to continue supporting python 2.4 syntax since many
people use tito on RHEL 5, which has python 2.4.

http://docs.python.org/dev/howto/pyporting.html#capturing-the-currently-raised-exception
add unit test and module compatibility lib

Add modules compat lib to support both python 2 and 3, and
add unit test to detect regressions.

Tito uses two modules that have changed significantly between
python 2 and 3:

* commands module merged into subprocess module
* ConfigParser module renamed as configparser
add unit test and use print function for python3 compat

Python 3 does not have the print statement, so convert all
print statements to print functions and add an ugly duckling
unit test to detect regressions.
add python 3.3 to travis-ci build matrix
avoid ImportError in test suite on python 3

python3 does not allow relative imports.

test/unit/__init.py__ and test/functional/__init.py__ already exist,
which means we can use absolute import on all versions of python
without resorting to http://legacy.python.org/dev/peps/pep-0328/
python 2.4 (rhel 5) does not support 'with...as' syntax

resolves from a73c90cb...

    Using Python 2.4
    Using nose 0.1
    Running tito tests against: /home/sandbox/src
    EEEE.Warning: spacewalk.releng.* namespace in tito.props is obsolete. Use tito.* instead.
    Warning: spacewalk.releng.* namespace in tito.props is obsolete. Use tito.* instead.
    ..............................F...........
    ======================================================================
    ERROR: Failure: SyntaxError (invalid syntax (fixture.py, line 159))
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "/usr/lib/python2.4/site-packages/nose/loader.py", line 381, in loadTestsFromName
        module = self.importer.importFromPath(
      File "/usr/lib/python2.4/site-packages/nose/importer.py", line 39, in importFromPath
        return self.importFromDir(dir_path, fqname)
      File "/usr/lib/python2.4/site-packages/nose/importer.py", line 86, in importFromDir
        mod = load_module(part_fqname, fh, filename, desc)
      File "/home/sandbox/test/functional/fetch_tests.py", line 27, in ?
        from fixture import TitoGitTestFixture, tito
      File "/home/sandbox/test/functional/fixture.py", line 159
        with open(os.path.join(full_pkg_dir, 'tito.props'), 'w') \
                ^
    SyntaxError: invalid syntax
docs: createrepo is needed for functional tests