Add npm scripts for start, build, lint, etc. - fixes #903

[jgautsch]

Fixes #903

I ran into this issue- where $ gulp serve or even $ gulp -v would fail. The root cause seemed to be that the gulp executable in my path was not the same version as the one in node_modules/.bin/gulp.

npm scripts have the convenient feature of looking in node_modules/.bin for executables, so using npm scripts to run gulp tasks obviates the need to have the correct gulp version globally installed.

With this change, starting is as simple as:

$ npm install
$ npm start

or with yarn:

$ yarn
$ yarn start

This is convenient, conventional, and similar to how other projects like create-react-app.

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[jgautsch]

I signed it!

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