Add coveralls to travis

This also adds a badge to README.md.

.travis.yml

@@ -6,4 +6,8 @@ go:
 matrix:
     fast_finish: true
     allow_failures:
-        - go: tip
+        - go: tip
+before_install:
+    - go get github.com/mattn/goveralls
+script:
+    - $GOPATH/bin/goveralls -service=travis-ci -v

README.md

@@ -4,6 +4,7 @@ Gzip Handler
 [![GoDoc](https://godoc.org/github.com/tmthrgd/gziphandler?status.svg)](https://godoc.org/github.com/tmthrgd/gziphandler)
 [![Build Status](https://travis-ci.org/tmthrgd/gziphandler.svg?branch=master)](https://travis-ci.org/tmthrgd/gziphandler)
 [![Go Report Card](https://goreportcard.com/badge/github.com/tmthrgd/gziphandler)](https://goreportcard.com/report/github.com/tmthrgd/gziphandler)
+[![Coverage Status](https://coveralls.io/repos/github/tmthrgd/gziphandler/badge.svg?branch=master)](https://coveralls.io/github/tmthrgd/gziphandler?branch=master)
 
 This is a tiny Go package which wraps HTTP handlers to transparently gzip the
 response body, for clients which support it. Although it's usually simpler to