Blend Mode not implemented for CCLabelBMFont

[rkachowski]

The blend mode property isn't implemented for the CCLabelBMFont class which is resulting in some errors in SpriteBuilder when deserialising. This property needs to be added to the class for consistency and my sanity.

[slembcke]

Now that I think about it, setting the blend mode on a CCLabelBMFont was broken in v3 too. You could set a blend mode (or main texture, or shader) on any CCNode subclass, though it was ignored on most of them. I split the rendering methods out into a new subclass in v3 so they would only exist on the types of nodes that used them.

Should be an easy fix though.