Add relationship between comment and article.
default value of type of Article, Comment is md now.
Remove type of Article and Comment. Finish utils.content_convert.

The syntax supported by blog is depending on the converter. It's
markdown now.
Can specify a converter when create a Article or Comment now.
Provide two util function for apply params to converter.
Add  blog.session module.
blog.session.SessionManager.create_session return a tuple containing key-session
Add session support for blog.handlers.BaseHandler
Override get_current_user in BaseHandler.
Override on_finish but not finish method for cleaning up.
Add a series of user query method.
Add LoginHandler (without template).