PHP-904 Implement MongoDate::__set_state

Fixes PHP-904
https://jira.mongodb.org/browse/PHP-904

Allows MongoDate objects exported via var_export() to be parsed.
var_export() is handy when creating test fixtures.

This is my first time contributing to any php extension. I think I got it right, but happy to fix anything or resubmit against a different branch. Thanks!