izettle-messaging: Format all Date fields in events as RFC3339

This is mostly for discussion, as it could be controversial. This makes the messaging library slightly more opinionated. But I like opinionated frameworks :)

What this change does is that all fields on events of type `Date` will get treated as being formatted according to RFC3339.
* Examples of this date format is `2001-12-23T03:05:06.123+0100`.
* This applies both to deserializing of json in received events, and when serializing outgoing messages.
* When producing json for outgoing messages, the time zone will always be UTC, so published messages will have their `Date`s formatted as `2001-12-23T02:05:06.123+0000`.
Merge pull request #34 from iZettle/messaging/format-dates-as-rfc3339

izettle-messaging: Format all Date fields in events as RFC3339