prefer `pyvenv` rather than `virtualenv` if `--pyvenv` is given
Remove `--pyvenv` option.

Use pyvenv only if virtualenv is not installed and there is pyvenv.
install setuptools and pip into pyvenv
add SETUPTOOLS_VERSION and PIP_VERSION
add EZ_SETUP_URL and GET_PIP_URL
renamed virtualenv() to venv()
set STATUS as non-zero even if the setuptools or pip have failed to
install into pyvenv
Merge branch 'pyvenv'
do not create `python` executable in `pyenv-virtualenv`

it will be created by `virtualenv` or `pyvenv`