Add drowning mechanic to players.

* Players can stay in liquids for 15 seconds before drowning.
* Drowning damage occurs every second.
* Sounds are played when taking drowning damage.
* Breath meter is shown on the HUD.

![bubbles](https://f.cloud.github.com/assets/2770254/786222/412f4108-eaa4-11e2-90d2-5d44d761658a.jpeg)