修改顶部大图链接
	修改：      app/controllers/projects_controller.rb
	修改：      app/controllers/questions_controller.rb
	修改：      app/views/layouts/legacy/project_12.html.haml
	修改：      app/views/layouts/legacy/project_14.html.haml
	修改：      app/views/layouts/legacy/project_16.html.haml
	修改：      app/views/layouts/legacy/project_18.html.haml
	新文件：    app/views/layouts/legacy/project_21.html.haml
	修改：      app/views/layouts/legacy/project_special_12.html.haml
	修改：      app/views/projects/_form_question.html.haml
	新文件：    db/migrate/20140130060626_add_banner_url_to_projects.rb

说明：
	20140128165854_add_banner_url_to_project.rb: 添加此migrate文件， 向 Table projects 中添加 banner_url column, as string
	layout/legacy/ 下面的四个文件中仅将原有的直接写入的网址替换成了current_project.banner_url
	project.html.haml：修改顶部banner链接指向，使用了link_to image_tag
	_form_question.html.haml：添加输入banner_url的表单项