add try+/else

This patch adds an optional 'else' clause to try+ (and very good test coverage of the feature). The idea is [stolen from python](http://docs.python.org/2/tutorial/errors.html#handling-exceptions). It is a surprisingly useful construct, in my experience, and I'd love to have it in clojure :)

The basic goal is to make it easier to avoid over-catching exceptions. I had this bug in my code, recently:

```
(try+
  (do-something!)
  (tell-someone!)
  (catch Object e
    (requeue-do-something!)))
```

... when (tell-someone!) threw, it caused (do-something!) to be erroneously requeued. Without a try+/else construct, I wound up hacking something like:

```
(let [threw? (atom false)]
  (try+
    (do-something!)
    (catch Object e
      (reset! threw? true)
      (requeue-do-something!))
  (when-not @threw?
    (tell-someone!)))
```

With a try+/else construct, I'd have the much clearer:

```
(try+
  (do-something!)
  (catch Object e
    (requeue-do-something!))
  (else
    (tell-someone!)))
```

Hope you'll consider the patch!