Allow a number of influences per vertex other than 2

Our animators want to be able to have 4 influences per vertex... The behavior of the parser will change based on a `influencesPerVertex` property on the root of the JSON object. If this property is not present, it will default to 2. This should be fully backwards compatible.

One thing I'm not certain of is whether this would require additional changes elsewhere in the engine? My model does not look quite correct when exported at max 4 influences, but I think it's an issue with my shader, and the mesh's shape is correct. I would appreciate confirmation on this.

![screen shot 2014-03-03 at 11 40 53 am](https://f.cloud.github.com/assets/1529387/2313014/a916fe38-a303-11e3-978b-189a9891c403.png)