Add configurable virtualenv python discovery

SublimePythonIDE will now try to automatically discover the python interpreter
in the project's virtualenv. By default, it will search for a directory named
'venv'. This is configurable with the `virtualenv_dir_name` setting.

If no virtualenv interpreter is discovered, it will fall back on the system
interpreter.

The `python_interpreter` setting still takes precedence over the virtualenv
interpreter discovery. Is this what we want ?
Implement virtualenvwrapper style venv path guess
validate proj dir before using
Merge pull request #42 from asfaltboy/venv-discovery

Venv discovery - add on martin's pr with support for virtualenvwrapper guessing