Initialize group child players when group player is created.

Can't use Player.paused - need to change
don't update children when paused
Newly initialised child players have null startTime

Why is this test failing

Demo is working but tests are broken

Shouldn't need to set startTime - setting currentTime should apply offsets.

Set currentTime of children when they are created.
Add test case for playing and immediately pausing.
add test for pause and seek. needs to be split up.
Split up pausing and pausing-then-seeking tests.
Child current time relative to parent current time
Merge pull request #261 from rjwright/GroupChildInit

Group child initialization