Redefined blocks now actually work as expected

Fixes #79. I'm not 100% sure that this doesn't break anything but from my tests it seems to be fine. Would be great if someone who knows ECT could make this more elegant.

Basically makes this work as you would expect:

`base.ect`
```html
<< block 'image' : >>
  <img src="111.png" />
<< end >>
<< include 'imageBox' >>
<< block 'image' : >>
  <img src="222.png" />
<< end >>
<< include 'imageBox' >>
```
`imageBox.ect`
```html
<div class="imageBox">
  << content 'image' >>
</div>
```

The above example previously would compile both `<< include 'imageBox' >>` to only the first block content ( `111.png ).

I'd imagine something like this could be used to make #50 work as well without too much pain.
That to say, since this always returns true then it should likely not return anything and any code relying on it be refactored.