Add optional development dependencies.

Helm and pyenv-mode.
Add helm and pyenv-mode settings.
Consider value of python-shell-virtualenv-path.

If user provide this variable then anaconda-mode will run under its
python version.  This allow anaconda detect packages installed in
this environment easily.
Add ert test case for virtualenv detection.
Restart anaconda_mode server automatically.

If user change python-shell-virtualenv-path variable next time he
call anaconda-mode command it will restart anaconda_mode process
in right environment.
Update documentation for new virtualenv feature.