add new export extract_pem_encoded_public_key and tests

* Provided a means to get the x509 PEM-encoded
  public key from a valid certificate.  Used by EC erchef
  to correctly provide public key  when GETing a user.
Merge pull request #14 from opscode/mp/user-endpoint

add function to get the public key (PEM-encoded) from a  x509 PEM-encoded certificate.