v0.18.0

• Blacklist problematic virtualenv environment variables (#49)
• Use python -m virtualenv to invoke virtualenv (#47)
• Making sure all Commands use run in __call__. (#48)
• Addition of session.notify. (#39)
• Refactor the list of sessions into a manifest class. (#38)
• Changed some instances of session to be plural (#37)
• Small documentation updates (#36)