Add additional fields to Book - lower priority issue

[BingSZou]

The following fields are useful for the book.

DifficultyLevel : int
Duration : int -- number of hours for this course
Description : string, VarChar(128)
Image: LongBlog
Content:
Reviews: int -- scale of 1 - 5
FileLocation: VarChar(28) -- location of the file.

Tag: string -- additional tag for the book