Add len limit to user's nickname and email at model.
More Clear Format.
query_filter_by as a common classmethod of BaseModel.
staticmethod -> classmethod
Move context object used by server level. Optimize the clean up process.

Now a part of objects in blog/context.py are in the top folder.
Now Application and Handler should use self.ctx to access a context object.
The server.py can control the context objects' creating, and it have a
more powerful way to clean up the context.