README improvements

@seancorfield, @michaelklishin This patch is mainly for the `require` instead of `use` suggestion I mentioned on Twitter, but I made a few other small tweaks as well:

* Use (require '[x.y :as z]) instead of (use x)
* Move today-at example into clj-time.core section
* Break up long lines

I worked through all of the examples in the README as if I were new to this library *and* Clojure. The examples with ns aliases aren't as pretty but I think demonstrating good practices outweighs aesthetics. Also, it's easier to follow in some areas where multiple namespaces are being used.

Looking forward to any feedback you guys might have.