adds raster support

Hey @rochoa take a look.

the way to create a raster thing is: 
```
cartodb.createLayer(map, {
          user_name: 'dev',
          type: 'cartodb',
          sublayers: [{
             sql: 'select * from pop',
             cartocss: '#pop { raster-opacity: 1.0; }',
             raster: true,
          }],
})
```

I don't know if use ``raster: true`` or just ``geom_type: 'raster'`` what do you think ?