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
Change form labels and add help text.
Remind user of environment before submitting form.
Tidy display of environment label in nav header.
Merge pull request #70 from alphagov/tweak-copy-and-guidance

Tweak copy and guidance