Stop use hardcoded virtualenv name in python commands.

This allow users specify virtualenv throw third party extensions,
and not depend on name conventions.
Globally ignore tox cache directory.
Merge pull request #265 from proofit404/hotfix/python-support

Stop use hardcoded virtualenv name in python commands.
Small doc fix
Make projectile-replace faster if possible

Will try to use ag, ack, git grep, and grep, in that order.

Closes #262.