Misc cleanups, mostly in date recipes.
Merge pull request #337 from clojure-cookbook/cleanups-trh

Misc cleanups, mostly in date recipes.
Additions/reversions to #337

* "a infinite" -> "an infinite".
* Keep +clj-time+, even in titles. We're holding off on production-level
  feedback on this. Luke and I expect it will be easier to remove these
  from titles rather than adding them.
* Re-write the 'unparse' solution paragraph. It was still a little
  awkward.
* Re-write first sentence of discussion's introduction in formatting
  date recipes. Also a little awkward.
* Put "unparsing" in quotes. I'm not sure if its a word at all, and its
  different enough to draw attention to the parallel between this word
  and +unparse+.
* +clj-time+'s -> ++clj-time++'s. Single pluses don't render properly in
  the book.