Support the built-in venv module on Python 3.4+

Add command line option --builtin-venv that enables the use of the
built-in venv module. It should only be used for Python 3.4 and later
that bootstrap the new virtualenv with setuptools and pip.
Merge pull request #40 from akheron/builtin-venv

Support the built-in venv module on Python 3.4+