Add optional key argument to add-profiling!.

This adds the option to profile a fn via a different logging key, and
not the default namespace.fn-name key.
What do you think about using multiple arglists for `add-profile!` instead of using variable arguments?

``` clj
(defn- default-key [var]
  (keyword (-> var .ns .name str)
           (-> var .sym str))

(defn add-profiling!
  ([var]
   (add-profiling! var (default-key var)))
  ([var k]
   (alter-var-root var (fn [f] (fn [& args] (profile* k #(apply f args)))))))
```

This makes it clear that there are two supported arglists, and makes it really clear what the one argument version does.

Let me know what you think…