fix 'as' syntax for exceptions - incompatible with python<=2.5
the json module is not available in versions of python older than 2.6, simplejson should be used in versions less than python 2.6
store the requirements in a requirements.txt file so they can be more easily installed using pip on travis
Merge branch 'bug/older_python_versions' into bug/old_python_versions
use unittest2 instead of plain unittest so that the tests run the same in Python2.4, Python2.5, and Python2.6 as they do in Python2.7