add "clobber" rake target

Rakefile.rb

@@ -5,6 +5,7 @@ rescue Bundler::BundlerError => e
   raise "Unable to Bundler.setup(): You probably need to run `bundle install`: " + e.message
 end
 require "rscons"
+require "fileutils"
 
 class Obj2D < Rscons::Builder
   def run(target, sources, cache, env, vars)
@@ -46,3 +47,9 @@ task :default do
     env.Program("gss", sources)
   end
 end
+
+task :clobber do
+  ["build"].each do |dir|
+    FileUtils.rm_rf(dir)
+  end
+end