add AJAX button

[TrilipuT]

Add AJAX button field. We have possibility to set own ajax_action. If not - field name will be used.

[kuliebiakin]

I think that it would be good to have a possibility to set JS callback on success response. What do you think @TrilipuT ?
And also remove attribute "id" from spinner, since it will be duplicated in case when there are more than one Button are used on a page.