Support a "no_server" flag in test config.

* Will use a running instance of rippled (possibly in a debugger).
* Modify all tests to respect the server_default value.
* Fail test if new account already exists and has a balance.
* README.md with instructions for advanced test debugging, particularly using no_server.

test/config-example.js

@@ -3,6 +3,7 @@
 //
 
 var path        = require("path");
+var extend      = require('extend');
 var testconfig  = require("./testconfig.js");
 
 exports.accounts = testconfig.accounts;
@@ -59,6 +60,11 @@ exports.servers = {
   }
 };
 
+exports.servers.debug = extend({
+  no_server: true,
+  debug_logfile: "debug.log"
+}, exports.servers.alpha);
+
 exports.http_servers = {
   // A local test server
   "zed" : {
@@ -67,4 +73,4 @@ exports.http_servers = {
   }
 };
 
-// vim:sw=2:sts=2:ts=8:et
+// vim:sw=2:sts=2:ts=8:et