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
* `stmt (statement)` (from IRC)
* `pat (pattern)` (not sure though...asked on IRC and got no reply)
* `matchers (lhs of the => in macro rules)` (from [manual](http://doc.rust-lang.org/rust.html#macro-by-example))
* `tt (rhs of the => in macro rules)` (from manual; `token tree` from IRC)