Adding simple slider field.

I saw that you started some slider field using jQuery UI but wrote my own using http://refreshless.com/nouislider/ (lightweight & flexible).

Current commit accepts range, step and unit parameter, like so : 

![captura de tela 2013-06-02 as 19 53 04](https://f.cloud.github.com/assets/1606655/597063/363e9618-cbd7-11e2-953f-41c54d822585.png)

And the results look like : 

![captura de tela 2013-06-02 as 19 53 41](https://f.cloud.github.com/assets/1606655/597066/4b7b7906-cbd7-11e2-9260-c67e72a902ac.png)

Be sure to check my coding structure and let me know if something should be written differently, I'll be glad to make it cleaner if needed.