Added clob-to-string to sql.utils

This would be very useful when dealing with JdbcClob fields. 

Example:
```clojure
(ns shoebox.models.photo
  (:require [korma.sql.utils :as utils]))

(defentity photos
  (transform (fn [{metadata :metadata :as v}]
               (if metadata
                 (assoc v :metadata (utils/clob-to-string metadata)) v))))
```