Prevent errors when seek()-ing an unstarted tween. (Fixes #56).

Disallow numbers less than 0 to be passed to seek(). (Fixes #57).

Squashed commit of the following:

commit 805a19fe966adf796ae1b9f7760c64155cc52b8e
Author: Jeremy Kahn <jeremyckahn@gmail.com>
Date:   Thu Nov 13 20:36:17 2014 -0600

    Prevent negative Tweenable#seek parameters.

commit 361fb2ff6cadd7c8940c97ff7038a76aa9132404
Author: Jeremy Kahn <jeremyckahn@gmail.com>
Date:   Wed Nov 12 21:54:31 2014 -0600

    Adds potential fix foor seeking tweens that have not yet begun.