add Builder class

lib/rscons.rb

@@ -1,2 +1,8 @@
-require "rscons/version"
+require "rscons/builder"
 require "rscons/environment"
+require "rscons/version"
+
+module Rscons
+  DEFAULT_BUILDERS = [
+  ]
+end

lib/rscons/builder.rb

@@ -0,0 +1,4 @@
+module Rscons
+  class Builder
+  end
+end