Prometheus monitoring for GitLab Pages

This starts of the prometheus monitoring for GitLab Pages, and
resolves #42

Point to check:
- Are the metric names good, keeping Prometheus' conventions in mind?
- Golang, general style etc
- Shouldn't I do some voodoo magic to import this in the library?

vendor/github.com/prometheus/client_model/NOTICE

+Data model artifacts for Prometheus.
+Copyright 2012-2015 The Prometheus Authors
+
+This product includes software developed at
+SoundCloud Ltd. (http://soundcloud.com/).

vendor/github.com/prometheus/common/AUTHORS.md

+Maintainers of this repository:
+
+* Fabian Reinartz <fabian@soundcloud.com>
+
+The following individuals have contributed code to this repository
+(listed in alphabetical order):
+
+* Björn Rabenstein <beorn@soundcloud.com>
+* Fabian Reinartz <fabian@soundcloud.com>
+* Julius Volz <julius.volz@gmail.com>
+* Miguel Molina <hi@mvader.me>

vendor/github.com/prometheus/common/NOTICE

+Common libraries shared by Prometheus Go components.
+Copyright 2015 The Prometheus Authors
+
+This product includes software developed at
+SoundCloud Ltd. (http://soundcloud.com/).

vendor/github.com/prometheus/common/model/model.go

+// Copyright 2013 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 model contains common data structures that are shared across
+// Prometheus components and libraries.
+package model