RFC info from the gsasl README (with newer RFCs added).

Implemented:
  ANONYMOUS (RFC 2245, RFC 4505)
  CRAM-MD5 (RFC 2195)
  DIGEST-MD5 (RFC 2831)
  EXTERNAL (RFC 2222, RFC 4422)
  GSSAPI (RFC 2222, RFC 4752, requires GSS, Heimdal or MIT Kerberos)
  LOGIN (non-standard)
  PLAIN (RFC 2595, RFC 4616)

Not implemented:
  GS2-KRB5 (RFC 5801, no CB, requires GSS, Heimdal, or MIT Kerberos)
  KERBEROS_V5 (experimental, requires Shishi)
  NTLM (non-standard, client only, requires Libntlm)
  SCRAM-SHA-1 and SCRAM-SHA-1-PLUS (RFC 5802)
  SECURID (RFC 2808)
