Feature/v0.11 support

node 0.12 is coming (really!) and we are updating our dependencies to get ready.

This PR removes some unused code that made compatibility with v0.11 harder, and then uses the compatibilty shim from bnoordhuis/node-heapdump to port to v0.11.

I suppose ThrowException() could be moved into compat.h / compat-inl.h and then you can just write it as e.g.:

```c++
Isolate* const isolate = args.GetIsolate();
Local<String> s = C::String::NewFromUtf8(isolate, m);
C::Isolate::ThrowException(isolate, s);  // class name TBD
```

Good idea / bad idea?