Ensure limiting behaviour is correct for groups.

[shans]

For example, 'end delays on groups work correctly' test in group-animation.js should have these lines at the end.

assert.equal(getComputedStyle(this.target).marginLeft, '4px');
tick(105);
assert.equal(getComputedStyle(this.target).marginLeft, '0px');

[rjwright]

It is true that seeking a group can cause the childrens' current times to extend past their active duration, but that is not the reason for this bug

For example, 'end delays on groups work correctly' test in group-animation.js should have these lines at the end.

assert.equal(getComputedStyle(this.target).marginLeft, '4px');
tick(105);
assert.equal(getComputedStyle(this.target).marginLeft, '0px');

See #375