Added support for setting CRL distribution point URI

[tsaarni]

Adds new field Certificate.CRLDistributionPoints which is an array of strings. When defined, CRL Distribution Points extension will be added to the certificate.

When using the Go API, it is a field in Certificate:

cert := Certificate{Subject: "CN=Joe", CRLDistributionPoints: []string{"http://example.com/crl.pem"}}

When using the command line and manifest file:

subject: cn=server
crl_distribution_points:
- http://ca1.example.com/crl.pem
- http://ca2.example.com/crl.pem

The generated certificate will have following extension added:

Certificate:
    Data:
        X509v3 extensions:
            X509v3 CRL Distribution Points:
                Full Name:
                  URI:http://ca1.example.com/crl.pem
                Full Name:
                  URI:http://ca2.example.com/crl.pem

Fixes #23