Add integer timestamps to attestation

[jot2re]

Per request by @JamesSmartCell we need to add an extra field to our IdentifierAttesations containing an easily readable timestamp and expiration time. This reason is that currently it follows x509 and hence is a string. This costs a lot to parse in Solidity.

[jot2re]

@JamesSmartCell @oleggrib
The Java code is updated and merged with this feature. The JS and Solidity might need to be updated when needed. However, so far the Java code is fully backwards compatible with what is currently in place.