Fix timestamp issues with Tweenable#seek. Fixes #60.

Squashed commit of the following:

commit bcb6650af1b2af2f0e2a5798034bcaefbfc74ab6
Author: Jeremy Kahn <jeremyckahn@gmail.com>
Date:   Thu Nov 20 21:11:21 2014 -0600

    Build 1.3.10.

commit 268a94441ee0d1f47de132196755474dbdd829fe
Author: Jeremy Kahn <jeremyckahn@gmail.com>
Date:   Thu Nov 20 21:06:30 2014 -0600

    Adds @SBoudrias's test for #60.

commit d955b18ae3f656037b6dfc0c6979ea7ccbd79076
Author: Jeremy Kahn <jeremyckahn@gmail.com>
Date:   Thu Nov 20 20:25:49 2014 -0600

    Clean up timeoutHandler a bit.

commit 23fa05beab4b525a6565013c11e63ca925c11e94
Author: Jeremy Kahn <jeremyckahn@gmail.com>
Date:   Thu Nov 20 20:22:58 2014 -0600

    Remove unused variable.

commit 810adef67fe0ae659f3940beadc7d138896e771f
Author: Jeremy Kahn <jeremyckahn@gmail.com>
Date:   Thu Nov 20 20:14:30 2014 -0600

    Make timestamp override logic for seek() work with regular tweens.

commit becb808c86f40dda39648b4e208a78fbf2b0285e
Merge: 2015805 0eb050a
Author: Jeremy Kahn <jeremyckahn@gmail.com>
Date:   Thu Nov 20 20:10:57 2014 -0600

    Merge branch 'master' of https://github.com/ZainManji/shifty into bugfix/timestamp-override

commit 0eb050a7d51f69d8d4b1f876475abe41184d2d57
Author: Zain Manji <zainkmanji@gmail.com>
Date:   Thu Nov 20 13:16:57 2014 -0800

    fixed issue with the calculation of the current time in timeouthandler in seek