support for SIGNATURE_ALGORITHMS hello extension (TLS-1.2 only!):
 - send by the client (hash algorithm * signature algorithm) list
 - evaluated by the server to use an appropriate hash algorithm

for now, the only signature algorithm (hardcoded!) supported is RSA
Merge pull request #59 from mirleft/signature_algorithms

support for SIGNATURE_ALGORITHMS hello extension (TLS-1.2 only!):