scala: 2.10.2 - Add option for creating Idea symlinks

Intellij Idea requires a specific directory structure for the Scala directory.
See http://stackoverflow.com/q/10827017/1880101 for a description of the
problem.  This commit adds an 'idea' option to the 'scala' recipe, which creates
the required directories and symlinks as described in
http://scalacookbook.blogspot.com/2012/09/scala-for-intellij.html.