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

plugins/available/pyenv.plugin.bash

@@ -1,8 +1,9 @@
 cite about-plugin
 about-plugin 'load pyenv, if you are using it'
 
-export PYENV_ROOT="$HOME/.pyenv"
-export PATH="$PYENV_ROOT/bin:$PATH"
+export PYENV_PATH=`which pyenv`
+export PATH="$PYENV_PATH:$PATH"
+
 [[ `which pyenv` ]] && eval "$(pyenv init -)"
 
 #Load pyenv virtualenv if the virtualenv plugin is installed.