context module will add clean expired session task to cron now.
render_string will use get_template_namespace now.
Add a series of User.have_user methods.
get_user method may raise a TypeError now.
User.nickname is unique now. And let have_user can receive user nickname.

Add a new method named have_user_with_nickname.
Add User._query_filter_by staticmethod and use it.
Add register Handler.
Change the comments format model.
Use basestring not string in model.
Change the comments format in handlers.py
Change docstring format in utils.py
Change docstring format in cron.py
Change docstring format in session.py
Fix bug that RegisterHandler won't render and set current user after commit.
Add UserInfoHandler.
Use utf8 charset to connect database server.
Use current_user in templates to represent the current user.