Adding kwargs support

Refactoring the PieVenv class to put more functionality into the venv…

… class

Improving help text

Handle stdout and stderr better in the DEFAULT_CMD_FN

Improving selection of the right python binary to execute within and …

…outside of a virtualenv

Adding has method to env

Adding some convenience methods to env to handle annoying features of…

… os.environ

Adding env context handler

Minor bug fixes

Adding some error handling abilities