Fix play/pause behavior

This fixes a number of bugs such that:
* We return null for the currentTime while waiting to pause at the API
level (but preserve the time internally for calculating the computed
timing etc.)
* We correctly handle pause() following by play() in quick succession
* We make sure a player's start time is always null while paused

Generated from:

commit 615861df4286c3eac35fcaf4f60a04e718402f71
Author: Brian Birtles <birtles@gmail.com>
Date:   Fri Oct 3 15:22:23 2014 +0900

    Fix play/pause behavior

    This fixes a number of bugs such that:
    * We return null for the currentTime while waiting to pause at the API
    level (but preserve the time internally for calculating the computed
    timing etc.)
    * We correctly handle pause() following by play() in quick succession
    * We make sure a player's start time is always null while paused