Fix tests, refactor some code

Hello,

So I needed your module, and I thank you for your work.
When I saw the tests were broken, I couldn't help but to look at the code. Well, I may have done too much, but I let you decide on that.

If you have some remarks about my coding style, please let me know!
If you want me to revert the change of location of the files, let me know as well!

List of changes:
* Moved both files `index.js` and `test.js` to their respective directories.
* Re-factored tests to use the same a function testing both stream and buffer modes
* Styling improvements for `index.js`
* Updated dependencies
* Improved the gulpfile, replaced deprecated code

Cheers,

Antoine