Since I expected to build in Windows support in the medium - long term, I wrote `PEBuild::Unpack` to use a strategy pattern like [`PEBuild::Transfer`](https://github.com/adrienthebo/vagrant-pe_build/blob/master/lib/pe_build/transfer.rb). However I only had time to implement the unix side of things. What do you think of moving the `Unpack` detection logic to `pe_build/unpack.rb` and add a `PEBuild::Unpack::Copy` class to do the heavy lifting? This way the archive class doesn't have to handle the unpack logic.