Change the timestamp data

This PR changes the timestamp of the HMAC faye credentials we generate in two ways:

1. It uses the iso8601 format, which allows us to easily parse them in Javascript
2. It makes the "timestamp" the time at which the credential was granted rather than the time at which it was generated. We can do this because the authenticity of the token is guaranteed by the HMAC and it is more convenient because we dont have to keep state between the rails server / js / faye server