Book chapters [WIP] [CR]

* Fun books now have 'chapters' that are analogous to charges (and indeed use the same variable)
  * Each time a book is read, one chapter is deducted
* When a book that has run out of chapters is read again, it only gives 1/4 of the original morale bonus - a book isn't as fun when you read it again.

Any book that doesn't have a chapter count (ie, skill books) but does have positive fun will not be affected.

Addresses #1823 by making it so you can't read that one sci-fi novel for years on end and never get tired of it.

I picked the numbers involved out of thin air, so I'd like some feedback before I take off the [WIP] tag:
* I went with "4 chapters per point of intelligence the book requires" to calculate chapter counts. Too much? Too few? Bad metric? What do you think?
* Feedback on the 75% morale bonus reduction is also appreciated.