Support string to date coercion

Dates get serialized to JSON as ISO-8601 date strings such as `'2014-10-19T02:24:42.395Z'` which also can be passed directly to the `Date` constructor.  This makes it easy to support automatically coercing a date `String` back to a `Date` object when validation occurs.  I've also tested this with isvalid-express.js and it works great in my node app!