Tutorial: macros

[oakmac]

We need a tutorial covering everything about macros in ClojureScript.

Macros in CLJS are tricky due to their relationship to Clojure and the build process. We need to create a definitive reference on how they work.

[shaunlebron]

interesting point here that was recently documented: https://github.com/clojure/clojurescript/wiki/Differences-from-Clojure#macros

macros can have the same names as functions without causing problems. should include this fact in the tutorial