Document Player requirements and make directories optional

[neophob]

• Make sure directories are not needed
• document what components the full-screen player must support

[tmrobert8]

If I might add to this - also hook sensors/items up via roles/tags to avoid naming issues

[pfiaux]

The next release includes a new helper .addPlayerWidget({...}) to simplify adding the player, updated documentation and updated player example.

The queue directory will also be optional, for now the root directory will remain required.

For now the sensor/switch/sliders need specific names, until a feature like role/tags (#112) is implemented this will be a limitation.