support for newer SSL certificate types

more and more CAs are using certificates signed using SHA256 or SHA512,
whereas go's X.509 support by default only supports certificates signed
with a SHA1 hash.

Importing the two hashes causes them to be registered and available to
use for x509 which in turn allows connecting to servers using one of
these newer certificates
Merge pull request #188 from pilif/support-more-ssl-cert-hashes

support for newer SSL certificate types