Added support for passing an X509s directly to RPs

 * Added overload of RelyingPartySpec.EncryptionCertificate accepting an
   X509 Certificate.
 * Added test to support the above.
Removed unnessecary DeploymentItem
Added support for specifying a X509 via thumbprint

 * Added RelyingPartySpec.EncryptionCertificateIdentifiedBy
 * Added overload of above to support defaults of CurrentUser/My
 * Atted test to confirm able to load
 * Refactored X509Ceterficate retreval code into common helper
Refactored overloads into optional params