Migrate from Distribute to Setuptools

Distribute was a fork of Setuptools, but the two have now merged into
Setuptools (since version 0.7). This patch replaces the old Distribute
bootstrap script `distribute_setup.py` with the latest Setuptools
bootstrap script `ez_setup.py`.

On systems with the new Setuptools, the build would fail with the
follwing error message:

    ValueError: A 0.7-series setuptools cannot be installed with
    distribute. Found one at /usr/lib/python2.7/site-packages

The new bootstrap script fixes this.