staging should delete after being done #20

Or perhaps there should be something in the instructions about it? I just made some binaries with make bdist-osx or the like, and discovered that staging/ has eighteen gigabytes in it! Given that once the process finishes, one shouldn't need all that, perhaps it should act like a temp folder - delete once done using.

Of course, this is from an uninformed perspective since I only "use" binary-pkg, so there may be a compelling reason to keep stuff around. But 18G according to du seemed like somewhat of an excess since dist/ exists.