add coverage

.istanbul.yml

@@ -0,0 +1,3 @@
+instrumentation:
+  root: lib
+  include-all-sources: true

.travis.yml

@@ -9,5 +9,5 @@ node_js:
   - '6'
   - '5'
   - '4'
-before_script:
-  - npm install grunt-cli -g
+after_success:
+  - npm run coveralls

README.md

@@ -1,6 +1,7 @@
 # nodemailer-build-attachment
 
 [![Build Status](https://travis-ci.org/killmenot/nodemailer-build-attachment.svg?branch=master)](https://travis-ci.org/killmenot/nodemailer-build-attachment)
+[![Coverage Status](https://coveralls.io/repos/github/killmenot/nodemailer-build-attachment/badge.svg?branch=master)](https://coveralls.io/github/killmenot/nodemailer-build-attachment?branch=master)
 [![Dependency Status](https://gemnasium.com/badges/github.com/killmenot/nodemailer-build-attachment.svg)](https://gemnasium.com/github.com/killmenot/nodemailer-build-attachment)
 [![npm version](https://badge.fury.io/js/nodemailer-build-attachment.svg)](https://badge.fury.io/js/nodemailer-build-attachment)
 
@@ -12,6 +13,7 @@ Based on [Andris Reinman](https://github.com/andris9)'s work in [buildmail](http
 
  - Node.js 4+
 
+
 ## Install
 
 ```