Error code subclasses

http://duktape.org/api.html#defines.3

* [x] Raise error subclass in eval error handler
* [x] Raise error subclass for "complex objects"
* [x] Raise error subclass for "unknown objects"
Any other ways of doing the singleton? This seemed the easiest in C.

``` ruby
Duktape::ComplexObject.instance # => returns the singleton
```

I don't think people will use the `.instance` method much. I'd just do

``` ruby
result.is_a?(Duktape::ComplexObject) ? nil : result
```