isolate xmlsec1 use

First swing at making additional crypto backends possible. If you want to discuss, we can talk tomorrow.

There are no test cases covering encrypt/decrypt, and it was a bit too involved for me to create easily at this time. Can you help out with that?

/Fredrik

PS. overall test coverage is 70%, sigver.py 66%.

pip install pytest-cov
cd tests
ln -s ../src/saml2 .
python -m pytest --cov-report term-missing --cov=saml2 .