Server config functions

This implements #99 

Form config priority: Runtime->JSON_form->global config
Element config priority: Runtime->JSON_element->JSON_form->global_config

A array called 'export_array' determines what config fields should be
exported to the client.

Elements will only submit a config array if they are changed from form
level/global level.

You can override elements on runtime using the set_config() and set_element_config() functions of a Ultraform object.