Initialize group child players when group player is created.

@shans @dstockwell 
I don't know if this is the best way to achieve this, but it fixes the group play-pause-seek problem and doesn't break any tests.

Previously, playing, pausing and seeking a group would produce a paused group player with correct current time and no children. With this change you get a fully populated group player, and the children have seeked current times too. 
Weren't we waiting for a start time before constructing the child players so that we could synchronize them?