Add framework for customizable WHOIS commands

With some additional changes by Ben McIlwain.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=145447136

javatests/google/registry/whois/WhoisHttpServerTest.java

@@ -71,6 +71,7 @@ public class WhoisHttpServerTest {
     result.requestPath = WhoisHttpServer.PATH + pathInfo;
     result.response = response;
     result.disclaimer = "Doodle Disclaimer";
+    result.commandFactory = new WhoisCommandFactory();
     return result;
   }