Add optional button to toggle preview

Hi, 
I've made some changes in  three main js files to optionally add a button in the help button group. This button show or hide the preview panel through ``` jQuery.fadeToggle() ``` function. I tried to keep your coding style and use of tabulation as much as possible. I made a few tests on two browsers namely: 
* [Chromium](http://www.chromium.org/) (Windows & Linux UA)
* [dwb](http://portix.bitbucket.org/dwb/)

Regards, 
Tlux

PS: Let me know if you want me to regenerate the .min version of the combined js.