add 2x 3x 4x for logos

Signed-off-by: Miao Wang <shankerwangmiao@gmail.com>

_includes/footer.html

@@ -28,7 +28,7 @@
       </div>
 
       <div class="col-lg-4 col-md-4"  align="center"> 
-        <img class="img-responsive" style="margin-top:5%" src="/static/img/logo-white.png" />
+        <img class="img-responsive" style="margin-top:5%" src="/static/img/logo-white.png" srcset="/static/img/logo-white.png 1x, /static/img/logo-white@2x.png 2x, /static/img/logo-white@3x.png 3x, /static/img/logo-white@4x.png 4x" />
       </div>
     </div><!--/row -->
   </div><!--/container -->

_includes/nav.html

@@ -7,7 +7,7 @@
         <span class="icon-bar"></span>
         <span class="icon-bar"></span>
       </button>
-      <a class="navbar-brand" href="/"><img src="/static/img/logo-small.png" alt=""/> 清华大学开源软件镜像站</a>
+      <a class="navbar-brand" href="/"><img src="/static/img/logo-small.png" srcset="/static/img/logo-small.png 1x, /static/img/logo-small@2x.png 2x, /static/img/logo-small@3x.png 3x, /static/img/logo-small@4x.png 4x" alt=""/> 清华大学开源软件镜像站</a>
     </div>
     <div class="navbar-collapse collapse navbar-right">
       <ul class="nav navbar-nav">