Added support for wheels

This adds support for bdist packages (wheels).

I've done this by factoring a `pip2archive` function out of `pip2tgz` and creating a `pip2whl` command that uses the same function to do the work.

I didn't change the default behavior of the `pip2pi` command to use `pip2whl` rather than `pip2tgz` because I figured that would involve dealing with backwards incompatibilities.  One could have `pip2pi` default to use `pip2whl` whenever the wheel package is available.  If you'd like to have that change I can certainly add it to this PR.

ref https://github.com/wolever/pip2pi/issues/21