Move to a src/ and dist/ based folder structure

Moved all source files to `src/` and all output files to `dist/`.

You might want to give this a try, works out really well in any project in my experiece.
Makes for a really clean repo, where only source files have to be comitted.
Also, `node_modules/` should be excluded from the git repo (added a line for this in `.gitignore`).

Cheers!