Implement Worker wrapper class

[krogenth]

Using the BWAPI::Unit wrapper interface from #7, we'll want to create a custom BWAPI::Unit wrapper specifically for workers. For now, workers can go to the nearest minerals to the starting location.

Workers will have several potential jobs to perform:

• mine minerals
• extract gas
• morph into new buildings
• scout

We may eventually implement some form of attacking as well, primarily for the event where an enemy has rushed us.