lend_book() finished

I've finished implementing (but not testing) the functionality in lend_book, so it should be possible to lend a book with any combination of information.

As a pleasant side effect, it's now possible to search for people (patrons).

I really need to break out a lot of the stuff in lend_book() to helper functions, because it's really cluttered.  But right now I was more concerned with getting the code written than anything.