added PolygonRegionLoader for the AssetManager

I felt like it may be useful, so I added the PolygonRegionLoader [from libgdx-utils](https://bitbucket.org/dermetfan/libgdx-utils/src/fe0bce159517de10c7aa097ae2e2069f88f7c09f/src/main/net/dermetfan/utils/libgdx/PolygonRegionLoader.java?at=default). I also added it to the default loaders of the AssetManager in its constructor.

The PolygonRegionLoader uses the com.badlogic.gdx.graphics.g2d.PolygonRegionLoader but is an AssetLoader. The line starting with "i " (can be configured using PolygonRegionParameters) in PSH files is used to figure out which Texture it depends on. If that line does not exist, a sibling of the PSH file with the same name and an image file name extension (like "png", "jpeg", ... can be configured) is searched.