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.
Maybe we should modify the comment here as well.