Rename 'registry_tool' to 'nomulus'

This changes everything with external visibility beyond the codebase
(i.e. the name of the compiled binary and the documentation that refers
to it). It does not change a lot of things internal to the codebase,
i.e. the "RegistryTool" class didn't change its name. We can rename that
in a subsequent CL if we want to.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=135022087

java/google/registry/tools/server/ListTldsAction.java

@@ -29,7 +29,7 @@ import google.registry.util.Clock;
 import javax.inject.Inject;
 import org.joda.time.DateTime;
 
-/** An action that lists top-level domains, for use by the registry_tool list_tlds command. */
+/** An action that lists top-level domains, for use by the {@code nomulus list_tlds} command. */
 @Action(path = ListTldsAction.PATH, method = {GET, POST})
 public final class ListTldsAction extends ListObjectsAction<Registry> {