Add support for reading multiple certificates from a PEM file.

[davidnarayan]

The current implementation does not support multiple certificates in a single PEM file. This PR adds support for reading multiple certificates from the PEM. This use case is necessary when the client needs to pass a complete certificate chain as part of a mutual auth transaction.

Fixes #144

Note: Sample private keys have been included as part of the test suite. They are example keys only and should never be used as any part of a production implementation.