Always use custom `Date.parse` method in `new Date()`

If es5-shim is loaded before a library that overrides `Date.parse` with an incorrect implementation (e.g., returning a `Date` object instead of milliseconds, as does [datejs](http://www.datejs.com/)), the shimmed `Date(isoString)` constructor can return an invalid or incorrect date.

To fix this, the `Date()` constructor will use the local, custom parse method defined by es5-shim, instead of using the [potentially overridden] global `Date.parse` method.