fixed loadPolygon center of mass

This fixes the problem of `loadPolygon` with the center of mass. See http://www.html5gamedevs.com/topic/5854-p2-collision-mesh-misaligned/?p=35121.

loadPolygon delegates the creation of the vertices to addPolygon (which itself uses p2's fromPolygon method) whenever it finds only a single convex shape. This is not necessary. The parsing in loadPolygon can construct 1..n vertices. 

I just removed the if branching around `data.length === 1` and it works as expected.

The problem with addPolygon itself is not fixed with this. As it's used at several other places in phaser like 
`createBody` or `createParticle` we should move the vertices creating logic from `loadPolygon` into `addPolygon`. So that loadPolygon only iterates over the data- this would be a clean solution. Let's use this as the first step, I need more time than I currently have to validate to desirable clean solution.

Regards
George

Testcase with the appropriate data (attached) present.
```javascript

var game = new Phaser.Game(800, 600, Phaser.CANVAS, 'phaser-example', { preload: preload, create: create});

function preload() {
  game.load.image('ship', 'assets/sprites/ship.png');
  game.load.physics('physicsData', 'assets/physics/ship.json');
}

var contra;
var start = false;

function create() {
  game.physics.startSystem(Phaser.Physics.P2JS);
  ship1 = game.add.sprite(400, 300, 'ship');
  ship2 = game.add.sprite(500, 300, 'ship');

  game.physics.p2.enable(ship1, true);
  game.physics.p2.enable(ship2, true);
  
  ship1.body.clearShapes()
  ship1.body.loadPolygon('physicsData', 'ship');
  
  ship2.body.clearShapes()
  ship2.body.addPhaserPolygon('physicsData', 'ship2');
}

function update() {

}

function render() {

}

```
![ship](https://cloud.githubusercontent.com/assets/1701755/2768813/e28869d2-ca49-11e3-904f-be5736970cc7.png)

ship.json
```javascript
	{ 
		
		"ship": [
        {
            "density": 2, "friction": 0, "bounce": 0,
            "shape": [
                16, 31,
                0, 29,
                16, 0,
                31, 29
            ]
        }
    ],

    "ship2":[
      {
        "isSensor": false,
        "filter": {
          "group": 0,
          "categoryBits": 1,
          "maskBits": 65535
        },
        "polygons":[ 
          [16, 31,0, 29,16, 0,31, 29]
        ]
      }
    ]

	}
```

![screen shot 2014-04-22 at 20 13 42](https://cloud.githubusercontent.com/assets/1701755/2768856/2ebcad2c-ca4a-11e3-9dd5-822a5dfacc54.png)