Allow signer to override subject data in CSR.

Currently, the CFSSL signing API uses whatever subject data is in the CSR. This pull request proposes adding a subject data parameter to the Signer type's Sign method, which contains the actual subject information that should be used for the certificate. In effect, supplying a value for subject data only uses the public key information from the CSR and uses the supplied subject data for the certificate.
and change doc here.