Track Date of Last Login

* add column type [timestamp] (http://www.h2database.com/html/datatypes.html#timestamp_type) named `dateLastLogin` to table User
 (mapped to `java.sql.Timestamp`, or supported by `java.util.Date`- we'll use whichever works better)
* update `User` class with corresponding getters and setters.
* create a method in class `Database` to update the timestamp column, and call it in `LoginAction.execute()`.