Problem: README build badges too generic

Solution: Show Linux, OSX and Windows labels on build badges for more
visibility on supported platforms.

Additionally added license badge.

Author: kurdybacha

README.md

@@ -1,6 +1,7 @@
-[![Travis-CI Status](https://travis-ci.org/zeromq/cppzmq.svg?branch=master)](https://travis-ci.org/zeromq/cppzmq)
-[![Appveyor Status](https://ci.appveyor.com/api/projects/status/bvi5axi8um7goh50/branch/master?svg=true)](https://ci.appveyor.com/project/zeromq/cppzmq/branch/master)
+[![Travis-CI Status](https://img.shields.io/travis/zeromq/cppzmq/master.svg?label=Linux%20|%20OSX)](https://travis-ci.org/zeromq/cppzmq)
+[![Appveyor Status](https://img.shields.io/appveyor/ci/zeromq/cppzmq/master.svg?label=Windows)](https://ci.appveyor.com/project/zeromq/cppzmq/branch/master)
 [![Coverage Status](https://coveralls.io/repos/github/zeromq/cppzmq/badge.svg?branch=master)](https://coveralls.io/github/zeromq/cppzmq?branch=master)
+[![License](https://img.shields.io/github/license/zeromq/cppzmq.svg)](https://github.com/zeromq/cppzmq/blob/master/LICENSE)
 
 Introduction & Design Goals
 ===========================