Allow overriding the download url for distribute.

To use this, set the env variable URL_BASE_DISTRIBUTE
to the new base url, prior to running bootstrap.sh.

i.e.

export URL_BASE_DISTRIBUTE=http://b.pypi.python.org/packages/source/d/distribute/
curl -sL https://raw.github.com/someuser/dev-tools/somebranch/bootstrap.sh | bash