Added package files. Updated README.

Added package.json and package-lock.json files that allow building the stylesheet. Changed gulpfile.js to work with latest versions of dependencies. Added three different tasks: build, watch and serve. Package has corresponding scripts defined. Proxy URL is no longer editable within the file itself, but could be defined on the package or environment level. README files have additional documentation to reflect these changes. All changes have also been propagated to the default subtheme directory.