add Rscons.clear() class method

lib/rscons.rb

@@ -22,4 +22,13 @@ module Rscons
 
   class BuildError < Exception
   end
+
+  # Remove all generated files
+  def self.clean
+    cache = Cache.new
+    cache.targets.each do |target|
+      FileUtils.rm_f(target)
+    end
+    Cache.clear
+  end
 end

lib/rscons/cache.rb

@@ -132,6 +132,11 @@ module Rscons
       }
     end
 
+    # Return a list of targets that have been built
+    def targets
+      @cache[:targets].keys
+    end
+
     # Private Instance Methods
     private