Feature/gulp

This PR builds from the gulp system started by @blazkovicz in https://github.com/mozilla/metrics-graphics/issues/222.
Because the /examples/ and /dist/ folders are now separate, a couple of modifications needed to be made to compile src files to both.

While doing so, I tried to improve the workflow by adding some build tasks:

gulp -- will compile js/css to /dist/ folder and /examples/ folder
gulp watch -- will run default and watch for changes
gulp serve -- will start a development server and run watch
gulp test -- will run test suite

I left the jslint task in, but not in the build process, if you want to lint as part of the build process we should open another ticket to properly configure it.