renamed templates/initializer.rb to install_weixin_rails_middleware.rb
updated README.md
added migration to add weixin secret key
removed token_column config
updated token_string and weixin_secret_key configures
updated comments for engine_path configure
added weixin_token column with migration
refactor configuration
changed routes weixin_token param to weixin_secret_key
added default weixin_secret_key
replaced token_model_instance with weixin_token_string
updated configurates
refactor configuration.rb
added token_model_callback after initialize
refactor weixin_authorize_helper
updated README.md