[api] Improve the .use() method. Use the memory engine by default

lib/nconf.js

@@ -19,7 +19,9 @@ nconf.stores = require('nconf/stores');
 // specified `type`.
 //
 nconf.use = function (type, options) {
+  if (!nconf.store || type.toLowerCase() !== nconf.store.type) {
     nconf.store = new nconf.stores.create(type, options);
+  }
 };
 
 //
@@ -121,3 +123,8 @@ nconf.key = function () {
 nconf.path = function (key) {
   return key.split(':');
 };
+
+//
+// Use the `memory` engine by default
+//
+nconf.use('memory');

lib/nconf/stores/file.js

@@ -22,6 +22,7 @@ var File = exports.File = function (options) {
 
   Memory.call(this, options);
 
+  this.type   = 'file';
   this.file   = options.file;
   this.format = options.format || JSON;
 };

lib/nconf/stores/memory.js

@@ -17,6 +17,7 @@ var nconf = require('nconf');
 //
 var Memory = exports.Memory = function (options) {
   options     = options || {};
+  this.type   = 'memory';
   this.store  = {};
   this.mtimes = {};
 };

lib/nconf/stores/redis.js

@@ -23,7 +23,8 @@ var async = require('async'),
 //       namespace:nested:key  ==> 'value'
 //
 var Redis = exports.Redis = function (options) {
-  options        = options || {}
+  options        = options || {};
+  this.type      = 'redis';
   this.namespace = options.namespace || 'nconf';
   this.host      = options.host || 'localhost';
   this.port      = options.port || 6379;