improve message generation

[wayneparrott]

package.json

• renamed binary script to generate-ros-messages

generate_messages.js

• minor change in console.log message

README.md

• updated to reference the generate-ros-messages script and how to run it using npx

test-generate-messages-bin.js

• added test to run generate-ros-messages using npx

test-interactive.js

• fixed minor lint issues.

fix #776