add gem and build badges

Author: cowboyd

README.md

@@ -1,4 +1,7 @@
 # libv8
+[![Gem Version](https://badge.fury.io/rb/libv8.svg)](http://badge.fury.io/rb/libv8)
+[![Build Status](https://travis-ci.org/cowboyd/libv8.svg?branch=master)](https://travis-ci.org/cowboyd/libv8)
+[![Build status](https://ci.appveyor.com/api/projects/status/ugeqff972kxlba5j?svg=true)](https://ci.appveyor.com/project/cowboyd/libv8)
 
 A gem for distributing the v8 runtime libraries and headers in both
 source and binary form.