Form actions patch

Currently to get different coloured buttons you had to have a something like `"type" => "danger"`in the `attr` array. Since all entries in attr are rendered into markup in `button_attributes` block, you end up with two `type` attributes in your markup.
This ButtonExtension introduces a new option `buttonclass` for that purpose.
In addition it also provides the possibility to render an `<a>`-tag as BS-styled button using the option `"asatag" => true` and having the `href` in the `attr` array.