Adds support to supply a return method for the button.
Fixes bug that the button choice wasn't saved.
Sets the return method value in the button output, if available.
Adds a placeholder to store help content for a coming help class.
Adds reference URL.
Removes duplication of return_method code.
Implements the return method in the admin settings.
Merge branch 'return-query-string' into develop. Fixes #16.