Make constructors just methods.

* Eliminate "new" reserved word.
* Allow "this" before a method definition to define a constructor.
* Only create a default constructor for classes that don't define one.

doc/site/index.markdown

@@ -14,7 +14,7 @@ a familiar, modern [syntax][].
       }
     }
 
-    var adjectives = new Fiber {
+    var adjectives = Fiber.new {
       ["small", "clean", "fast"].each {|word| Fiber.yield(word) }
     }