Allow multiple PEM-encoded certificates in the ca_file.

fixes #167

This should be what I want. This is pretty common practice: Some examples I found offhand of software that accepts a CA file and decodes multiple PEM-encoded certs:

- `openssl verify`'s `-CAfile` option
- mutt's `certificate_file` option
- curl's `--cacert` option
- puppet's `ssl_server_ca_auth` and `ssl_client_ca_auth` options