add optional kwargs to rsa export key, and fix some pep8 warnings


Added optional **kwargs to exportKey which can include prot_params 'iteration_count' 
and 'salt_size', key_params, and randfunc. And added unittest testExportKey16() to 
test_import_RSA.py

And fixed some PEP8 warnings in RSA.py.

Fixed unittest in previous closed pull request to address requirements...
New code and unittests work with python >= 2.1 per requirements.  No more 'with' introduced in Python 2.5