Branch/tdg units

@eric79 @youngca 
Here's the first pass of getting unit icons so that they can be added to the map.
I get this now, but it confused me at first.  Could you put a comment to the effect of:

    // This node is the unit unless its type is *modifier*, in which case its parent node is the actual unit node.
    // The case where it is a modifier is handled in the conditional below.