Updates documentation for added callback attribute

* Updates JS files to new version that adds callback attribute
* Updates customisation forms to add callback field
* Adds small explanation of callback setup to documentation
Merge pull request #25 from cbetta/callback_option

Updates documentation for added callback attribute
Added Required label next to Required fields.  Added placeholders for all fields.
Updated dist files
Clarifies that buttons can be previewed

Copy was not very clear that you can preview and try the buttons.
Copy now reflects that a bit more.
Would still like to have the previews on by default but haven't figured out how.
Merge pull request #26 from cbetta/preview_clarification

Clarifies that buttons can be previewed