#9 Possibility changing class name of span containers

[Sergey Lavrov]

It will be cool to have possibility changing class name of span containers from default cppcms_form_help / cppcms_form_input / cppcms_form_error to user defined like it can be done for id / name and other attributes.

For example when using twitter bootstrap css framework it will be usefull set different span class name for submit button to form-actions.