Add PostgreSQL notifier (#2739) (#2824)

* The user is required to specify a table name and database connection
  information in the configuration file.

* INSERTs and DELETEs are done via prepared statements for speed.

* Assumes a table structure, and requires PostgreSQL 9.5 or above due to
  the use of UPSERT.

* Creates the table if it does not exist with the given table name using
  a query like:

    CREATE TABLE myminio (
        key varchar PRIMARY KEY,
        value JSONB
    );

* Vendors some required libraries.

vendor/vendor.json

@@ -77,6 +77,18 @@
 			"revision": "467733eb9c209042fb37d2074a5b6be33a529be0",
 			"revisionTime": "2016-07-06T21:06:00+02:00"
 		},
+		{
+			"checksumSHA1": "dNYxHiBLalTqluak2/Z8c3RsSEM=",
+			"path": "github.com/lib/pq",
+			"revision": "50761b0867bd1d9d069276790bcd4a3bccf2324a",
+			"revisionTime": "2016-08-31T22:25:20Z"
+		},
+		{
+			"checksumSHA1": "xppHi82MLqVx1eyQmbhTesAEjx8=",
+			"path": "github.com/lib/pq/oid",
+			"revision": "50761b0867bd1d9d069276790bcd4a3bccf2324a",
+			"revisionTime": "2016-08-31T22:25:20Z"
+		},
 		{
 			"path": "github.com/mattn/go-colorable",
 			"revision": "9056b7a9f2d1f2d96498d6d146acd1f9d5ed3d59",