add unit tests and fix exception clause compatibility

Exceptions are now compatible with both python 2.5 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 2.5 syntax since many
people use tito on RHEL 5, which has python 2.5.

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