Add package.json

This file is used by npm to manage packages. Its inclusion here should
lower the barrier to use this code.

Change-Id: I65c0ee0b02faa727931f13f385fb2f470bae676d
Reviewed-on: https://gerrit.causes.com/23159
Tested-by: Joe Lencioni <joe.lencioni@causes.com>
Reviewed-by: Henric Trotzig <henric.trotzig@causes.com>