Use an existing virtualenv if it exists

This is useful if you use virtualenv wrapper and are currently in a
virtualenv with the requirements installed when you run start_app.sh
Use a default port if it is not specified

This is defined in the Readme - makes sense to assume to use the default
if not specifying a different port
Merge pull request #68 from alphagov/improve-start-app-script

Improvements to start app script