Enhance YamalManager to Load related assets by name.

I changed the YamalManager so it can load related assets by name. I updated the yamal example to load a font and use it.  An example of use:

    !Sprite
    Name: Bob
    #Load zombie texture
    Texture: !asset zombie

Some minor changes to ContentManager were required, hope doesn't make it a breaking change.

Note: I was unable to get the WIN8METRO build to work (must be missing something extra).  There was only 1 line different in the ContentManager that was WIN8METRO, but my changes it look good to me.