Fix idiomatic subclasses of Pjs classes #backcompatBreaking 

Prior to this commit an idiomatic subclass of a Pjs class would fail to inherit the constructor, for example:
```js
function Subclass(info, moreInfo) {
  // ...
  var arg1 = slightlyChange(info), arg2 = drasticallyChange(moreInfo);
  // ...
  Superclass.call(this, arg1, arg2);
}
```
would silently create and discard a new instance of `Superclass` and not mutate as `this` as intended.

Of course, if you're writing your idiomatic subclass of a Pjs class by hand, you can just do `this.init(arg1, arg2)` and you're golden.  However, in [CoffeeScript][1] and even [ECMAScript 6 (Harmony)][2], `super(arg1, arg2)` compiles to `Superclass.call(this, arg1, arg2)`, and if you don't define a `constructor`, there's an implicit `Superclass.apply(this, arguments)`, which it's pretty lame to break.

[1]: http://coffeescript.org/#classes
[2]: http://wiki.ecmascript.org/doku.php?id=strawman:minimal_classes#super

The fix is a one-liner:
```diff
     function C() {
-      var self = new Bare;
+      var self = this instanceof C ? this : new Bare;
       if (isFunction(self.init)) self.init.apply(self, arguments);
       return self;
     }
```
The only back-compat break is jayferd#9: `this.constructor(arg1, arg2)` will also mutate `this`, being fundamentally equivalent to `C.call(this, arg1, arg2)`. But it occured to me that this isn't a problem for idiomatic classes because to instantiate them you do `new this.constructor(arg1, arg2)`, which we can make work for Pjs classes too, and just say that in that ambiguous case you use `new` to "force" instantiation. Note that for varargs you just do `this.constructor.apply(null, argsList)` as ever. (Btw we totally forgot to suggest `this.constructor.call(null)` in jayferd#9.)

I really think that given the tradeoff of being a well-behaved class system vs the edge case of a factory method, especially considering how easy it is to just use `new`, we should favor being well-behaved.

The minified size does increase a smidgen, but fret not, I saw some ways to reduce minified size that I'll open separate PRs for. (Avoiding the repeated `this` intro'd in this change does not in fact reduce minified size, I tried that.)