Added build system with linting and also added support of different evironments (node)

This PR helps to reduce manual effort and it adds `gulp` build system which does:

- generates `poll.js` and `poll.min.js`;
- replaces `@@VESRION` attribute with related attribute from `package.json`;
- runs lint utility (`ESLint`);

Also I have refactored some code to support different environments and not it is possible to run on `nodejs`.