Adding try/catch clause

### try/catch 

Adding try/catch clouse like a http://en.wikibooks.org/wiki/Clojure_Programming/Concepts#Exception_Handling

```clojure
(fn exception ()
  (try
    (do (throw (new Exception "Custom")) NULL)
    (catch Exception e (-> e (getMessage)))))
(check (exception) "Custom")
```

### class

- For extends Exception class, I made the fix of `class-extends` clause.
- For more simple class added support blank class. 

```clojure
(class-extends OtherException Exception)
```

### syntax suggestion

- Make support the class extends most like clojure http://en.wikibooks.org/wiki/Clojure_Programming/Concepts#User-Defined_Exceptions

```clojure
(class OtherException :extends Exception)
(throw (new Exception "Custom"))
```