Add framework for customizable WHOIS commands

With some additional changes by Ben McIlwain.

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

java/google/registry/whois/WhoisCommand.java

@@ -17,7 +17,7 @@ package google.registry.whois;
 import org.joda.time.DateTime;
 
 /** Represents a WHOIS command request from a client. */
-interface WhoisCommand {
+public interface WhoisCommand {
 
   /**
    * Executes a WHOIS query and returns the resultant data.