78 encrypted assertion

Implementation of issue #78.

Summary
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+ Refactored certification reading into helper class. 
+ Created custom exceptions for certificate handling. 
+ Created helper / static class to decrypt an EncryptedAssertion replacing the contents of the Saml2Response with the decrypted contents.

Questions
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Due to the dependency on the Windows certificate store, I'm unsure how to proceed with writing unit tests without creating an unnecessary abstraction. The previous CertificateElement class didn't handle this code coverage either... are you OK with this?

Also unsure on how to proceed with testing of message decryption. If full testing is desired, then we need a test helper able to generate EncryptedAssertion messages. Does anything exist yet?