Updated the documentation

Eugene Burmako

I have just updated documentation for macros. It now features a getting started example and a reference of macro context and reflection API.

The documentation also features my recent talk about metaprogramming in Scala 2.10. Be sure to take a look, because the slides not only describe trees, symbols and types - three core data structures of Scala compiler, but also suggest a few tricks that make it easy to explore compiler's inner workings and find out necessary information by yourself.