SRP6 methods to calculate and verify the evidence messages M1, M2 and for the session Key

I added java support for the missing parts of the standard SRP-6 protocol
M1 = H(A,B,S)
M2 = H(A,M1,S)
Key = H(S)

@see also:  http://www.bouncycastle.org/devmailarchive/msg13267.html