Simplify check if inheriting from a non-Pjs class

If proto.init is truthy but not a function, like 1 or something, you'll
get a

    TypeError: Object 1 has no method 'apply'

which I feel like is excellent behavior.

If we're inheriting from a Pjs class but `proto.init` is overridden to a
falsy value, we'll think we're inheriting from a non-Pjs class and set
`proto.init` to the superclass constructor, which isn't the most
defensive, fail-early thing to do, but is also what happened prior to
this commit.

Really, to be defensive in our non-pjs check, we should check
`('init' in proto)`, so if `proto.init` is overridden to anything
but a function it'll TypeError like above.

README.md

@@ -43,7 +43,7 @@ tom.move()
 
 ## how is pjs different from X
 
-Most class systems for JS let you define classes by passing an object.  P.js lets you pass a function instead, which allows you to closure private methods and macros.  It's also <0.4kb minified (`make report`: 498).
+Most class systems for JS let you define classes by passing an object.  P.js lets you pass a function instead, which allows you to closure private methods and macros.  It's also <0.4kb minified (`make report`: 478).
 
 ### why doesn't pjs suck?
 

src/p.js

@@ -57,11 +57,9 @@ var P = (function(prototype, ownProperty, undefined) {
         }
       }
 
-      // if there's no init, we assume we're inheriting a non-pjs class, so
-      // we default to applying the superclass's constructor.
-      if (typeof proto.init !== 'function') {
-        proto.init = _superclass;
-      }
+      // if no init, assume we're inheriting from a non-Pjs class, so
+      // default to using the superclass constructor.
+      proto.init = proto.init || _superclass;
 
       return C;
     })(definition);