Added better support for pyenv, virtualenvwrapper, and autoenv
working in concert with one another.
Features added:
- Added a pyenv-virtualenv plugin which creates virtual environments
  using the current pyenv python version
- `cd` into a directory that has a virtual environment already created
  and autoenv will automatically activate that environment
- Current virtual environment is displayed in command prompt
Changed pvenv functions to avoid name conflicts
Run `pyenv virtualenvwrapper` and remove virtualenv-init cmd
Added pyenv-init to allow pyenv virtualenvwrapper to init properly
Skip workon if not in root directory
Merge pull request #341 from lucasnad27/master

Added pyenv-virtualenv plugin