Compiler: re-adding extension appends it to the end of the list.

This is an attempt to solve problem of extension dependencies, see http://forum.nette.org/en/21383-extension-dependencies-feedback for more details.
Thanks to #30 core Nette extensions can now be (re-)added using `ExtensionsExtension`. So to solve the problem mentioned in the forum you would use configuration something like:
```
extensions:
    presenters: Kdyby\PresentersLocator\DI\PresentersLocatorExtension
    inject: Nette\DI\Extensions\InjectExtension
```
Or possibly `PresentersLocatorExtension` could re-add `InjectExtenion` in its `loadConfiguration` method.
is the unset neccesary?