api/bucketPolicy: Use minio-go/pkg/set and fix bucket policy regression. (#2506)

Current master has a regression 'mc policy <policy-type> alias/bucket/prefix'
does not work anymore, due to the way new minio-go changes do json marshalling.
This led to a regression on server side when a ``prefix`` is provided
policy is rejected as malformed from th server which is not the case with
AWS S3.

This patch uses the new ``minio-go/pkg/set`` package to address the
unmarshalling problems.

Fixes #2503

vendor/vendor.json

@@ -107,6 +107,12 @@
 			"revision": "db6b4f13442b26995f04b3b2b31b006cae7786e6",
 			"revisionTime": "2016-02-29T08:42:30-08:00"
 		},
+		{
+			"checksumSHA1": "A8QOw1aWwc+RtjGozY0XeS5varo=",
+			"path": "github.com/minio/minio-go/pkg/set",
+			"revision": "9e734013294ab153b0bdbe182738bcddd46f1947",
+			"revisionTime": "2016-08-18T00:31:20Z"
+		},
 		{
 			"path": "github.com/minio/miniobrowser",
 			"revision": "2e74c097f04fd7927d46fee6482d41e7b5cbf830",