Added buttons as a default to drupalgap_form_defaults().
#118, #119. Changed comment_edit(), node_edit(), taxonomy_form_vocabulary(), taxonomy_form_term(), user_login(), user_register(), user_profile_form() to expect form and form_state to be passed into their builder functions.
Changed drupalgap_mvc_model_create_form() to call drupalgap_form_defaults() and then append its elements and buttons rather than create the form from scratch.