Ignore failure of pyenv activate

When the current pyenv environment is not a virtualenv, this line
exits with a return code of 1. This is annoying for those of us
who report the last return code in our own PROMPT_COMMAND, so
ignore the output of `pyenv activate` if we're not currently using
a virtualenv.

- Add in script to detect the virtualenv file
- Add in the ability to read a python virtualenv file
- Add in a script to show the current virtual environment name
- Tidy up the comment block in some of the scripts
- Update the activate script to use the pyenv-virtualenv-name script
- Add in a script that explains how the virtual env is set
- Add in a helper script to write to the virtualenv file
- Update the pyenv-virtualenv-prefix script
- Update the pyenv-virtualenv-init script

fix wrong output in test in fish, refs 240a1de

"(any command)" is not supported in test in fish, we have to set a
variable and then use that

fix warns for virtualenv-init corner case on fish

The situation is when in a directory containing python binary,
`pyenv prefix' will return empty (caused by `pyenv which' returning
"./python"). This change adds quotes for `test', otherwise `test' will
warn about missing argument.

Some doc fixes: typos, grammar, clarification and formatting

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"