RSA sign/verify and hash generation functions

[Murray S. Kucherawy]

Looks like signing is now working, but I can't import a DER public key with this:

	gnutls_x509_crt_init(&pubkey);
	key.data = <pointer to DER data>;
	key.size = <length of DER data, currently >;
	gnutls_x509_crt_import(pubkey, &key, GNUTLS_X509_FMT_DER);

The import call is returning -73, GNUTLS_E_ASN1_TAG_ERROR.

What I'm actually importing is an DER encoded RSA public key, namely a PEM one with the "---BEGIN PUBLIC KEY---" removed and then the base64 decoding done.  It's a 1024 bit key, and post-decoding it's 162 bytes long.