add public_url option

test/setup.js

@@ -8,7 +8,8 @@ before(function() {
   process.chdir('test_data');
   var running = require('../src/server')({
     configPath: 'config.json',
-    port: 8888
+    port: 8888,
+    publicUrl: '/test/'
   });
   global.app = running.app;
   global.server = running.server;

test/style.js

@@ -23,6 +23,7 @@ describe('Styles', function() {
           res.body.sources.should.be.Object();
           res.body.glyphs.should.be.String();
           res.body.sprite.should.be.String();
+          res.body.sprite.should.equal('/test/styles/test-style/sprite');
           res.body.layers.should.be.Array();
         }).end(done);
     });