[#117] Replaced Make with gulp

Replaced Make with gulp.
The following build options are now available.
- `npm run build` - uses browserify to consolidate all js files in `./lib` and creates a `numbers.js` and `numbers.min.js` in `./src`.
- `npm run format` - uses jsbeautifier to format all js files in `./lib` and `./test`
- `npm run lint` - run jshint against code in `./lib` and `./test`
- `npm run test` - run test cases in `./test` using mocha.

Reference: https://github.com/sjkaliski/numbers.js/issues/117