Entity package

This is the final design of Entity, I and Jun decided to remove some useless function such as user as well as comment. Consequently, there are only 5 classes left, which are persistent 5 table in database.
Knot represent the knot entries in database, Fixture represent the objects such as tree, hook or loop for specified knot. Line represent the strings for specified knot, one knot has one, may be two(or more?) strings. Step represent the status or snapshot for particular string, the core functionality is show the steps of knot. Point store the sequence of point for particular step of line.