Remove `python-virtualenv` which was no longer used.
Change the installation path of the `virtualenv.py` script.
s/VIRTUALENV_CACHE_PATH/PYENV_&/g
make installation path of virtualenv.py configurable
Download `virtualenv.py` if specified version has not been installed.
update README
Merge branch 'virtualenv-install'
restore missing `VIRTUALENV_PYTHON_BIN`
(1) fix handling of hooks
(2) remove $VIRTUALENV_PATH after failed creation
Unset environment variables which starts with `VIRTUALENV_`. These variables are reserved for virtualenv.