Consider relatively positioned elements for slot height

* fixes #765
* adjustments in SlotContents to have the full power of
  Arrays at our fignertips without extending it all the time..
  maybe better suited as a simple delegator though

I'll leave some in code comments. :) Result looks good:

![screenshot from 2014-06-25 16 21 36](https://cloud.githubusercontent.com/assets/606517/3386456/ac9c25c2-fc74-11e3-8bbc-8d92eaf71b74.png)

I like avoiding the repetition, but would it make more sense for `SlotContents` to just derive from `Delegator` pointing to the underlying `@contents` array?