trillian_log_signer command

The trillian_log_signer binary runs the process which sequences new entries, integrates them into the corresponding log, and, finally, creates a new LogRoot with updated root hash.

The naming of this binary originally came from the fact that it would also sign each of the new LogRoots, but that functionality was removed. Renaming the binary would likely cause some surprise for existing log operators and so the decision was taken to leave it as-is for now.