add licence banner

[rjw57]

To be enterprise-ready, all source files must include a licensing banner to
allow for IP audits.

[dchest]

Thank you for your contribution(s)!
Unfortunately, we can't accept it unless you sign a CLA and send it to /dev/null.
You can find the CLA and instructions here: https://github.com/dchest/semicolon-js/blob/master/CLA.txt

[rjw57]

I've sent the CLA to our legal team. However, being on Windows, they have only managed to send it to NUL:. I hope that's satisfactory.