[WP8] Do not reload .XNBs of types we don't need to reload

Some types don't need to get reloaded, like for example SoundEffects. By
excluding those from ReloadAsset() we can speedup resuming.

Things to consider:
-SoundEffect is the only type I use in my active project that doesn't
contain GPU resources. Other types can be added to the list.
-Make the HashSet publicly accessible so users can add their own types.
-Instead of an Exclusion Set we can use an Inclusion Set  (Texture2D,
Texture3D, Cube, Model, etc)