SCT signature verification, encode/decode and accessor functions

I know the log returns base64-encoded data and so a one-stop "create SCT from log response" is not necessarily a bad idea. But I think it shouldn't be the only way to construct an SCT, i.e., we should also have

SCT_set_version(unsigned char version);

etc

and those should not operate on base64-encoded data.

You do not have to add those in this change. But maybe rename this one to something more verbose, e.g., SCT_set_from_log_response?

Also, reusing structures only leads to trouble, so maybe an

SCT *SCT_new_from_log_response(....);

is even cleaner?