Added section '13.11.3 Adding Compile-Time Methods using AST'

Implementing AST transforms in a plugin is tricky, because the AST classes must be precompiled. Thanks to http://reinhard-seiler.blogspot.com.au/2011/09/grails-with-ats-transformation-tutorial.html I was able to get this working. This commit adds a section to the grails guide covering AST transforms.