Vendor pkg_resources and _markerlib from setuptools 2.0.2
Switch to importing the vendored pkg_resources
Remove the setuptools version check

With the vendoring of pkg_resources we no longer need to worry
about the version of the installed setuptools.
Have a nice error when setuptools is not available
Add setuptools bundling to the changelog
Work around python 2.6 not having ``python -m``
Merge pull request #1434 from dstufft/vendor-pkg-resources

Vendor pkg_resources