Define StudentIndentificationCode back to the Student resources

The 3.1 model made drastic changes to the attributes of Student. 

This change is necessary to have the 2.6 id look up ported to be supported in the 3.x version.

StudentIdentificationCode exists when a student is associated with some organization that assigns the identifier. Each identifier may be issued under the authority of different organizations: SEA, LEA, School, a government entity (e.g., SSN, Drivers License, Passport), or even a vendor application (e.g., SIS, security badging system). The alternatives are:
Place the identifiers in associations to the various organizations. This has the disadvantage of scattering identifiers across many different associations and may require defining organizations that would not normally be included in the model.
To support searching for a specific identifier, the Identifiers could be broken out into its own entity. However, today the IdentificationSystem descriptor semantics are too loose to support the important matter of unique identifiers. Geoff has suggested a URI scheme that would be more precise with the proper guidance.