add a test data file parser

This simple format will be used to store multiple syntaxes of the same message
so that test suites for different languages can reuse them
pyloxi: factor out test utility functions
pyloxi: start using data files for test cases

For the moment I'm just replacing the test functions one by one instead of
iterating over all test data files. We should think about doing that but I'm
not sure how to make failures easy to debug.
pyloxi: test packing the unpacked object

This should give us a little protection from faulty __eq__ methods.
Merge pull request #12 from rlane/test-data

Share test data between backends