Fix test case for Date.prototype.valueOf().

This test case failed when run in a UTC timezone.  Use Feb 1, 1970 as the
date instead, so that the result of valueOf() is truthy no matter what
timezone you are in.
Add 'npm test' and travis CI integration.
Add testling integration.

This allows us to automatically test es5-shim against a variety of
different browsers.
Add travis badges.
Add testling badge.