Move code from lib/ to src/

Also add copyright notice to main file, and update stale
node-cssjanus urls in some places (package.json was already updated).
build: Add .npmignore file

Don't package the configuration and scripts for building and testing.
Add !important support for four-value rules, with tests

The regex change was given by Bartosz Dziewoński at
https://bugzilla.wikimedia.org/show_bug.cgi?id=61440#c1 .

Fixes #23