Rework builder interface to only use #run method - close #91

The builder's #run method will be called repeatedly until it returns
true or false. The Builder#wait_for method can be used to cause a
builder to wait for a Thread, Command, or another Builder.

build_tests/json_to_yaml/Rsconscript

@@ -10,7 +10,7 @@ build do
         end
         @cache.register_build(@target, :JsonToYaml, @sources, @env)
       end
-      @target
+      true
     end
     env.JsonToYaml('foo.yml', 'foo.json')
   end