[security-services]Base64 in core and bindings

Title: [security-services]Base64 in core and bindings

I notice that neither core-28 nor bindings-12 have a reference for base64.

First of all, note that it is base64, not base-64 or base 64.

The reference is RFC 2045, which is here: http://www.ietf.org/rfc/rfc2045.txt.

This is sufficient for artifact encoding, but it seems to me that in schemas, we should using the built-in data type base64Binary. Among other things this has a whitespace facet that deals with the fact that RFC 2045 says that base64 requires a line break after no more than every 76 characters. This affects keys, certificates, Kerberos tickets, etc. which are likely to be longer than 456 bits. 

Hal