Download bower packages as zips

@devoncarew

Addresses #2999.

The fact is that Bower _always_ uses GitHub as the backend storage. So the only use case for this capability to switch between the two mechanisms is to see how they compare, after we perhaps find a way to significantly speed up our git. It can also be a reference code later if we find a use case for pulling some other packages (not Bower) from some other backend git storage.