title: add an admin user for a specific database only in mongodb
body: <p>On Mongodb 2.6.4 I want to create an admin user for a specific database.</p>
  
  <p>I tried this:</p>
  
  <pre><code>use myapp-dev
  db.createUser(
    {
      user: "myapp-dev-admin",
      pwd: "xxx",
      roles:
      [
        {
          role: "userAdmin",
          db: "myapp-dev"
        }
      ]
    }
  )
  </code></pre>
  
  <p>It appeared to create the user, and I could auth with <code>db.auth('myapp-dev-admin', 'xxxx')</code></p>
  
  <p>Except he can't do anything on the db.</p>
  
  <pre><code>&gt; show dbs
   2014-10-30T02:11:51.376-0700 listDatabases failed:{
  "ok" : 0,
  "errmsg" : "not authorized on admin to execute command { listDatabases: 1.0 }",
  "code" : 13
  } at src/mongo/shell/mongo.js:47
  &gt; show collections
   2014-10-30T02:12:02.672-0700 error: {
  "$err" : "not authorized for query on myapp-dev.system.namespaces",
  "code" : 13
   } at src/mongo/shell/query.js:131
  </code></pre>