Add boost scripts

* Add Meter.AddAir()
* Make Collectable run OnBoost() on all boosts attached to the object
Hook up air boost script with first crystal
Make AirBoost.airMeter private

* Find it by game object name in Start()
Change air/crystal bonus ratio

* Used to start with 40 and gain 4, now start with 20 and gain 8
Merge pull request #1 from jakeboxer/boosts

Add an air boost for picking up crystals