SAMLAttributeStatement serializatin and SaveBootstrapContex config

•Extented ToXElement extension methods to support  SAMLAttributeStatment  
•Support for  SaveBootsrapContext  config element in  CreateClaims()  method

To be correct, this should be named "Stub" and not "Mock". A mock is the more intelligent version of a stub. A stub is just anything that makes it run. A mock should also verify the caller behaviour. I'll do the rename when merging.