Allow the download of the bootstrap script to be skipped. Fixes #517.

Either passing `--skip-bootstrap-download` to `setup.py build` or by having `SKIP_BOOTSTRAP_DOWNLOAD=1` as an environment variable.
Automatically download the bootstrap script when running `sdist`. Fixes #517 and #520.
Merge pull request #521 from s0undt3ch/issues/517

Allow the download of the bootstrap script to be skipped. Fixes #517.