Added is_exclusive modifier to pick questions.

Adds an omit checkbox and omit_text checkbox to the pick questions.
Checking the omit checkbox adds a "none of the above" choice
(default) to the answers.  If "none of the above" is no good,
user can override the text with the omit_text field.
	modified:   app/models/permitted_params_decorator.rb - adds omit,
	omit_text.

	modified:   app/views/questions/_answer_fields.html.erb - adds
	new omit, omit_text form fields.

	modified:   lib/assets/stylesheets/surveyor_gui/surveyor_gui.sass -
	omit formatting.

	modified:   lib/surveyor_gui/models/question_methods.rb - adds params
	for omit and omit_text to the build_complex_question_structure call.

	modified:   lib/surveyor_gui/models/question_type_methods.rb - adds
	omit and omit_text to the _process_answers_textbox method, and modifies
	the _create_an_answer method to take any additional parameters with arg.