performance: gjson parsing for readXLMeta, listParts, getObjectInfo. (#2631)

- Using gjson for constructing xlMetaV1{} in realXLMeta.
- Test for parsing constructing xlMetaV1{} using gjson.
- Changes made since benchmarks showed 30-40% improvement in speed.
- Follow up comments in issue https://github.com/minio/minio/issues/2208
  for more details.
- gjson parsing of parts from xl.json for listParts.
- gjson parsing of statInfo from xl.json for getObjectInfo.
- Vendorizing gjson dependency.

vendor/vendor.json

@@ -169,6 +169,18 @@
 			"revision": "2e25825abdbd7752ff08b270d313b93519a0a232",
 			"revisionTime": "2016-03-11T21:55:03Z"
 		},
+		{
+			"checksumSHA1": "+Pcohsuq0Mi/y8bgaDFjb/CGzkk=",
+			"path": "github.com/tidwall/gjson",
+			"revision": "7c631e98686a791e5fc60ff099512968122afb52",
+			"revisionTime": "2016-09-08T16:02:40Z"
+		},
+		{
+			"checksumSHA1": "qmePMXEDYGwkAfT9QvtMC58JN/E=",
+			"path": "github.com/tidwall/match",
+			"revision": "173748da739a410c5b0b813b956f89ff94730b4c",
+			"revisionTime": "2016-08-30T17:39:30Z"
+		},
 		{
 			"path": "golang.org/x/crypto/bcrypt",
 			"revision": "7b85b097bf7527677d54d3220065e966a0e3b613",