Issue: Nested Books

One source of confusion in the past is that Books are 1-dimensional. By adding a single INT database column to the vault_book table, it's possible to place Books inside other books for multi-dimensional hierarchies.

On the data saving side, this would only require an additional field in the UPDATE command for books. On the user side, this would require a change to the way the chapter list is constructed. The chapter query would be slightly modified and an ibookcache (a la iforumcache) would need to be formed on the fly for the Table of Contents to lay out correctly.

This is an appropriate change at this time because the general Table of Contents needs to be reworked anyway to properly render the Book TOC.