doc: add npm badge and use flat square style

PR-URL: #162
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

Author: joyeecheung

README.md

@@ -1,6 +1,7 @@
 # llnode
 
-[![Build Status](https://travis-ci.org/nodejs/llnode.svg?branch=master)](https://travis-ci.org/nodejs/llnode)
+[![npm](https://img.shields.io/npm/v/llnode.svg?style=flat-square)](https://npmjs.org/package/llnode)
+[![Build Status](https://img.shields.io/travis/nodejs/llnode.svg?style=flat-square)](https://travis-ci.org/nodejs/llnode)
 
 Node.js v4.x-v8.x C++ plugin for the [LLDB](http://lldb.llvm.org) debugger.