Texture

Added some changes to Texture and BaseTexture that we discussed in #71.

BaseTexture now has a static `PIXI.BaseTexture.fromImage` method, which the static `PIXI.Texture.fromImage` method will use under the hood. I also made the Texture Object smart enough to accept a normal texture as the source, so you can do:

```js
var texture = PIXI.Texture.fromImage('/some/image/url.png'),
    texture2 = new PIXI.Texture(texture);
```