Serialize the AuthnEvent when adding to session

[decentral1se]

• Any changes relevant to users are recorded in the CHANGELOG.md.
• The documentation has been updated, if necessary.

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This takes a naive approach to solving this issue:

1. I use the __dict__ attribute to take the useful data from the AuthnEvent object
2. Use the dict access syntax for the session['authn_event'] from there onwards
3. Rebuild the object with AuthnEvent(**session['authn_event']) to use the valid() function
   3a. I add new kwargs to the AuthnEvent __init__

Another approach would be to use the jsonpickle package, but I was thinking this was a nice simple approach without the new dependency.

This will close off #360.

[coveralls]

Coverage increased (+0.01%) to 62.94% when pulling a5cc187 on lwm:serialize-authn-event into 747c0f7 on OpenIDC:master.