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/typical/echo_command_string.rb

@@ -1,7 +1,7 @@
 class MyBuilder < Rscons::Builder
   def run(options)
     @env.print_builder_run_message("MyBuilder #{@target}", "MyBuilder #{@target} command")
-    @target
+    true
   end
 end