Add CAS 3.0 attribute support, and style cleanups

[carsongee]

@fmyzjs, here is a cleanup of an attribute mapper that does 2 and 3. Would you mind trying it out with your protocol 3.0 server just to verify? I took your code but switched it to using in instead of the has_key it had before.

[fmyzjs]

Thanks all .I will do it as quickily as i can

[bdero]

Did you happen to try this out, @fmyzjs ?

[pdpinch]

Were you ever able to test this @fmyzjs ?

[pdpinch]

@carsongee What should we do with this PR and mitocw/django-cas#2