Fix #398
Use a new CSS class 'vjs-disabled' to disable display of the volume controls. This allows custom styles the flexibility to use methods other than `display:block` to layout the controls.
Update tests
Check for the presence of `vjs-disabled` instead of whether the display is set to none for volume and mute control tests.
Rename vjs-disabled to vjs-hidden
Disabled is a bit overloaded with visible but unusable controls, so use hidden instead. Re-arrange default styles a bit and use dom helper methods instead of the vjs-objection functions.
Merge pull request #399 from dmlap/issue398

Fix #398