Implement BWAPI Unit wrapper interface

[krogenth]

We'll want units to handle orders in potentially special ways, therefore we will need wrapper classes for our Player Units to better implement such requirements.

For now, this class should:

• store the final target destination
• store the BWAPI::TilePosition queue they are need move to reach the final target destination
• the Unit's id

functionality:

• pure virtual function to determine if the Unit is busy
• pure virtual function to manage the Unit's orders
• capability to draw the Unit's next destination/final target destination