Raise an exception if this arg looks like a timestamp

[shalecraig]

In the case where someone calls Timecop.freeze(some_timestamp), they probably meant to freeze at the provided timestamp, instead of that many seconds in the future.

I don't think I'd want to ship this PR, but can massage this PR into whichever of these you prefer:

1. if arg > k, using that timestamp
2. if arg > k, materializing an exception that can be caught
3. Something else?

---

k

I chose 1300000000 somewhat arbitrarily. Happy to change it to something like 1000000000 instead.

Tests

Happy to write tests once you let me know which outcome you prefer.

[tom-lord]

they probably meant ...

I chose 1300000000 somewhat arbitrarily ...

:-1: It's a nice idea, but I don't think this should be merged...

This patch may be useful for your specific use case, but I don't think the gem should be "cluttered" with this hardcoded value, or even as a configuration option.

If you passed a timestamp into the method, then that's a bug in your code; not a bug in TimeCop.