Update for Prometheus Community

Add standard Prometheus build setup
* CircleCI config
* Makefile
* Go modules
* Golang-CI Lint
* promu config
* Remove /vendor
* Remove mage build
* Update READMEs

https://github.com/prometheus-community/postgres_exporter/issues/478

Signed-off-by: Ben Kochie <superq@gmail.com>

cmd/postgres_exporter/pg_setting.go

@@ -1,3 +1,16 @@
+// Copyright 2021 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 package main
 
 import (