Merge pull request #270 from shootingatshadow/aes-support

Add AES support for PIV General Authenticate

 adds new routine sc_right_trim in sc.c and opensc.h. It is used by  PIV card driver when using the piv-tool.
Remove hardcodes from Mutual Authenticate

Support nonces that are not only 8 bytes in
Mutual Authenticate. Use the witness length
to determine the nonce size, thus existing
systems using 8 bytes will continue to use 8
bytes. However, with AES 256, the nonces could
be a single block size of 16 bytes or greater.