support filtering inactive proxy targets

for @rgbkrk
closes #1
> Isn't Javascript great?

You have no idea. I had a `try/catch` block here (because failure to parse would obviously raise, right?) and was wondering why it wasn't raising. Of course, `Date.parse` returns NaN on invalid input, and `Date(NaN)` *doesn't* fail; it creates a special `Invalid Date` object, for which `isDate()` returns `true`. Amazing.