SNAACertificate implementation

Here is a pull request for SNAACertificate implementation, also including a client library and an organization service implementation.

At the higher level, we have added "client-certificate" and "organization-service" folders.
In the "snaa" folrder, we have added a package called "eu.smartsantander.cea.certificate".

In the "snaa" folder, we have added a "certs" subfolder, giving examples for using organization certificates.