Add 2 tasks to conditionally add more settings.

Settings will be added to path set with
`virtualenvwrapper_shell_rc_file` if the variables are set:

1) virtualenvwrapper_venvs_project - to set a PROJECT_HOME environment
variable
2) virtualenvwrapper_venvs_python - to set VIRTUALENVWRAPPER_PYTHON with
a python path other then the default one

README was updated accordingly.