Initial delivery of metrics code (#81)

* Initial delivery of metrics code

* Fix build issues

* Fix build issue with go vet

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

@@ -0,0 +1,12 @@
+# Common
+[![Build Status](https://travis-ci.org/prometheus/common.svg)](https://travis-ci.org/prometheus/common)
+
+This repository contains Go libraries that are shared across Prometheus
+components and libraries.
+
+* **config**: Common configuration structures
+* **expfmt**: Decoding and encoding for the exposition format
+* **log**: A logging wrapper around [logrus](https://github.com/sirupsen/logrus)
+* **model**: Shared data structures
+* **route**: A routing wrapper around [httprouter](https://github.com/julienschmidt/httprouter) using `context.Context`
+* **version**: Version information and metrics