ADDED: Subject Alternate Names support for certs

Hi! I've been using your awesome cookbook lately, but for some of my servers, I needed to be able to have Subject Alternate Names support in my certs. Rather than asking you to add it, I figured it would be a good way to get better at Chef to add the feature myself. I'm submitting the PR to you in case you'd like to add it back into your cookbook.

Thanks!

It is now possible to create certs that support multiple DNS names.

ADDED: Tests for Subject Alternative Name feature
UPDATED: README.md now includes documentation for the SAN feature