Adds method to verify a certificate's signing chain

[achingbrain]

I frequently find myself having to validate a given certificate by verifying that it has a valid trust chain and as such that I possess all CA certificates in that signing chain.

Openssl gives us openssl verify -CAfile /path/to/ca.file /path/to/cert.file to do this so this PR adds a function to call openssl verify, docs and some tests to verify that a certificate does indeed have a valid signing chain.

So:

// a CA certificate
var ca = '-----BEGIN CERTIFICATE-----\nU2xvmeQANB0r9V...';

// an intermediate CA, signed by `ca`
var intermediate = '-----BEGIN CERTIFICATE-----\nWqvYOhA8uIpkNc...';

// a certificate signed by `intermediate`
var cert = '-----BEGIN CERTIFICATE-----\zodfM1CAZMCBE...';

pem.verifySigningChain(cert, [ca, intermediate], function (error, valid) {
  console.info('cert chain was valid?', valid);
});

Happy to discuss, make changes, etc.