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.

spec/rscons/builders/simple_builder_spec.rb

@@ -1,12 +0,0 @@
-module Rscons
-  module Builders
-    describe SimpleBuilder do
-      let(:env) {Environment.new}
-
-      it "should create a new builder with the given action" do
-        builder = Rscons::Builders::SimpleBuilder.new({}) { 0x1234 }
-        expect(builder.run(1,2,3,4,5)).to eq(0x1234)
-      end
-    end
-  end
-end

spec/rscons/environment_spec.rb

@@ -230,7 +230,7 @@ module Rscons
     describe "#find_finished_thread" do
       it "raises an error if called with nonblock=false and no threads to wait for" do
         env = Environment.new
-        expect {env.__send__(:find_finished_thread, [], false)}.to raise_error /No threads to wait for/
+        expect {env.__send__(:find_finished_thread, false)}.to raise_error /No threads to wait for/
       end
     end