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/custom_builder/Rsconscript

@@ -5,7 +5,7 @@ class MySource < Rscons::Builder
 #define THE_VALUE 5678
 EOF
     end
-    @target
+    true
   end
 end