More OTCrypto work, plus preparing for Nym changes

Signed-off-by: FellowTraveler <F3llowTraveler@gmail.com>
Abstracting OTMint/OTToken + OTCrypto. Also, I
renamed OTMasterKey to OTCachedKey.

Signed-off-by: FellowTraveler <F3llowTraveler@gmail.com>
Merge branch 'credentials'