Add getLabel() to native `Date` object.

Currently getLabel() will return NaN if an incorrect Date/ Date
object.  If valid, will return a string representing the date in
this format:  "MM/DD/YY".

There can be more thought behind this, possibly supporting a text
string that we can parse to display the results however the user
wants.

But till then, :shipit:
Now will extend the native Date object.

Added loop to add to object.  If defineProperty is supported, will
set as its prototype and make it non-enumerable.