Add --no-setuptools and --no-pip options.

In specialised circumstances (e.g., when testing new versions of distribute
or pip) it can be useful to create a virtualenv which does not include these
packages pre-installed. Add options to specify that when creating the
virtualenv.
Merged with upstream
Merge pull request #336 from pfmoore/disable-pip

Add --no-setuptools and --no-pip options.