Add support for verifying SHA-256 hash of Subject Public Key Info (#31)

For HTTP public key pinning (RFC7469), the SHA-256 hash of the Subject
Public Key Info (which usually only changes when the public key
changes) is used rather than the SHA-1 hash of the entire certificate
(which will change on each certificate renewal).

ssl/crypto_misc.h

@@ -78,6 +78,7 @@ struct _x509_ctx
     RSA_CTX *rsa_ctx;
     bigint *digest;
     uint8_t *fingerprint;
+    uint8_t *spki_sha256;
     struct _x509_ctx *next;
 };