Include a copy of markerlib

[embray]

markerlib provides a reusable implementation of distutils2 environment markers, and can be used to add support for that feature in d2to1 as well. It was also recently merged into distribute for Metadata 1.2 support, but we can't rely on users having a copy of it installed. To avoid complications it should probably ship bundled with d2to1 rather than listed as a dependency. It has an MIT license so this should be no problem.