Implement support for additional trust anchors on OSX.
Adopted code from `test_root_certs_mac.cc` to modify anchor certificates in `SecTrustRef` using `SecTrustSetAnchorCertificates`.
Required for rogerwang/node-webkit#2561
Merge pull request #24 from fancycode/additional_trust_anchors_osx

Implement support for additional trust anchors on OSX.