Add support for writing dz format to zip container (#402)

To enable this you can either use the `.zip` or `.szi` file extensions
or use `.tile({container: 'zip'})` with the `.dzi` extension.

package.json

@@ -58,6 +58,7 @@
   },
   "devDependencies": {
     "async": "^1.5.2",
+    "bufferutil": "^1.2.1",
     "coveralls": "^2.11.9",
     "exif-reader": "^1.0.0",
     "icc": "^0.0.2",
@@ -66,7 +67,7 @@
     "mocha-jshint": "^2.3.1",
     "node-cpplint": "^0.4.0",
     "rimraf": "^2.5.2",
-    "bufferutil": "^1.2.1"
+    "unzip": "^0.1.11"
   },
   "license": "Apache-2.0",
   "config": {