Refactor + new supported OS

Hi @rombert,

I've added support for Windows through Snarl and I've also created a factory so that we can easily add and instantiate new notifiers.

You can use the following images to attach to the wiki so that the README.md file can display them once interpreted by GitHub:

![maven-desktop-notifier-windows-failure](https://f.cloud.github.com/assets/170911/1351122/9bacb882-371a-11e3-9556-930256360119.png)
![maven-desktop-notifier-windows](https://f.cloud.github.com/assets/170911/1351123/9bad887a-371a-11e3-9d41-cae7925fc1b2.png)