1.在用户注册页面添加了email字段，并附带验证;

2.统一了username和email字段的唯一性验证方法。
[ci skip]

template/oj/account/register.html

@@ -14,6 +14,10 @@
                 <label for="real_name">真实姓名</label>
                 <input type="text" class="form-control input-lg" id="real_name" name="real_name" placeholder="真实姓名">
             </div>
+            <div class="form-group">
+                <label for="email">邮箱地址</label>
+                <input type="email" class="form-control input-lg" id="email" name="email" placeholder="邮箱地址">
+            </div>
             <div class="form-group">
                 <label for="password">密码</label>
                 <input type="password" class="form-control input-lg" id="password" name="password" placeholder="密码">