More conversion functions

This contains 3 sets of changes:
* Add -> functions for reading/writing CRLs to PEMs
* Add -> functions for working with a single certificate
* Move the issued-by? and has-subject? out of the API and into the tests

Looks like BC's getCRL() on the converter could throw a `java.security.cert.CRLException` in some cases here as well?  May be worth documenting it?