Add a SAN extension parser.

This is a generic SAN extension parser that comes from other projects
(viz., nss). Common practice is to embed this parser in your own
project, I'm not sure why, but I'm following the lead of other projects
like pyOpenSSL.
Add a method to load multiple certificates.

Useful, e.g., when loading a ca-bundle.crt file.
Add a method for decoding SAN names.
Add pyasn1 dependency.
Add tests for new san code.