Performance tuning for finding injector/resettor

Instead of recursively visit every class in the class inheritance hierarchy twice for both calls `inject` & `reset`, do it only one.

Note that there are few ways to do [this](https://github.com/nguyentruongtho/butterknife/blob/3f03ae41c6066ace1eeed3e19b900db3b61d9731/butterknife/src/main/java/butterknife/ButterKnife.java#L293-293) but the selected one is considered to be most suitable (generated class always has a naked constructor) and fastest, according to my dummy benchmark [here](https://gist.github.com/nguyentruongtho/b221292d4a52baed0ff2)
This `else` seems like a problem (and maybe it was the reason for not doing this earlier). I think this makes it impossible to do:
```java
class Foo {
  @InjectView(1) View foo;
}
class Bar extends Foo {
  @InjectView(2) View bar;
}
```
We can work around this by using composition rather than inheritance. The generated code keeps an instance of the superclass injector:
```java
class Bar$$ViewInjector implements Injector<Bar> {
  private final Injector<Foo> parent = new Injector<Foo>();

  @Override public void inject(Foo foo, ...) {
    parent.inject(foo, ...);
    // ...
   }

   // same for reset...
}
```