Building with gulp

A bit more convenient  way for developing with `gulp watch`.
Also, gulp is very fast, because it doesn't need to save intermediate files into `.tmp` directories.

Other changes:
- fixed `qunit.css` link in `demo/index.html`
- updated `README.md` with development process changes

I tried to re-create old build process so we still output all the files to `lib` and `lib_test` directories, so it shouldn't break anything. Someone please take a look and test if everything looks fine.