Refactor GeneralName/GeneralNames

[reaperhulk]

These two classes are functional but ugly. I'm considering the following refactor (input appreciated):

Take R509::ASN1::GeneralName and refactor it into a base class. From that base class subclass each type I want to implement. (e.g. rfc822name, dnsname, directoryname, uri, and ip for now).

This would simplify each class and make them more easily testable. The tag to type and type to tag mappings might have to be rethought.

GeneralNames is really just a hash with a few helper methods for adding/accessing items + a serialization method. It may not need to change, but I'm listing it here as a reminder that a refactor touching GeneralName may open up a better way to do the GeneralNames object.