Add gembox feature

tasks/mruby_build_gem.rake

 module MRuby
+  GemBox = BasicObject.new
+  class << GemBox
+    def new(&block); block.call(self); end
+    def config=(obj); @config = obj; end
+    def gem(gemdir, &block); @config.gem(gemdir, &block); end
+  end
+
   module LoadGems
+    def gembox(gemboxfile)
+      gembox = File.absolute_path("#{gemboxfile}.gembox", "#{MRUBY_ROOT}/mrbgems")
+      fail "Can't find gembox '#{gembox}'" unless File.exists?(gembox)
+      GemBox.config = self
+      instance_eval File.read(gembox)
+    end
+
     def gem(gemdir, &block)
       caller_dir = File.expand_path(File.dirname(/^(.*?):\d/.match(caller.first).to_a[1]))
       if gemdir.is_a?(Hash)