Macros: Split in topics, use playpen and cover more topics

I've rebased #185 and done a few changes. I'm satisfied with the current examples and the contents, but the wording could be further improved.

We are still missing the topic on how to export macros to be able to close #110 

@mdinger I hope you can review the changes
It does make sense to only explain the common ones but having most of them blank here looks really weird. A brief description might be in order. Also, that **Macro by example** [page](http://doc.rust-lang.org/rust.html#macro-by-example) still doesn't describe them so linking to it won't be of much help.