Update read to include pyenv-init - note.

It is necessary only if user want automatic fashion of activation.
If the users of pyenv-virtualenv doesn't need the feature, they can
omit the configuration safely.

Install script of setuptools-6.1/pip-1.5.6 will create those links if needed

added instructions for deleting virtualenvs

Do not allow whitespace in VIRTUALENV_NAME

While this could work in general, it fails in the end, because
whitespace is not allowed/supported in the shebang line with the
scripts in the created virtualenv.

Fix weird bash code issue which made bash complain about "then"

Use `PYENV_DEACTIVATE` to store which virtualenv has been deactivated (fixes #32)

The `pyenv-virtualenv` only takes care about the name of executables of `python`,
`setuptools` and `pip`.

Remove `--upgrade` feature since it is less useful