Add timeline support

[domhofmann]

This is a no-brainer and could be relatively easy, since we already have the ability to create multiple instances of PRTween. An open question is what syntax might look like. Ideally, it would be as simple as:

PRTween *timeline = [[PRTween alloc] init];
[timeline addToTimeline:
    [PRTween tween:someView property:@"alpha" from:1 to:0 duration:2],
    [PRTween wait:1],
    [PRTween tween:someView property:@"alpha" from:0 to:1 duration:2],
nil];