Djangosaml2

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- Allow defining custom config loaders. They can be dynamic depending on
the request.
- Do not automatically add the authentication backend. This way
we allow other people to add their own backends.
- Support for additional attributes other than the ones that get mapped
into the User model. Those attributes get stored in the UserProfile model.

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- Fix a crash in the idplist templatetag about using an old pysaml2 function
- Added a test for the previous crash

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- Upgrade pysaml2 dependency to version 0.4.1

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- Upgrade pysaml2 dependency to version 0.4.0 (update our tests as a result
of this)
- Add logging calls to make debugging easier
- Use the Django configured logger in pysaml2

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- Freeze the version of pysaml2 since we are not (yet!) compatible with
version 0.4.0

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- Avoid a crash when reading the SAML attribute that maps to the Django
username