Make RichDate an Enrichment of java.util.Date

I welcome comments on this.

This makes RichDate an "enrichment" meaning that we never return it from methods, or keep it as a parameter, but only use it to add methods to java.util.Date.

Issues:
* creates some minor breakage for users
* java.util.Date is actually mutable. Should we work so hard to standardize on that? Maybe this is the totally wrong direction: RichDate stores a Long timestamp and has implicits back to Date.

Just a sketch idea. Don't merge without discussion.