Find safe callables in user's code

Write a function that using static analysis decides whether given function/method is safe to be run. Err on the side of false-negatives, we don't want to accidentally execute user's code. Safe means "without side-effects or with side-effects we can handle", so this definition will be changing along with Pythoscope capabilities. Starting point for implementing this is locating callables that don't call anything else (remembering about implicit calls in Python, like __getattr__ or __add__).