SAMLAttributeStatement serializatin and SaveBootstrapContex config

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

I prefer using explicit `for each` loops instead of `ForEach`. Especially coupled with linq expressions it is error prone when maintaining the code (although this line is completely safe thanks to the `ToList()`).