Add C++ support

[calder]

No description provided.

[tedsuo]

This is definitely useful, but should be created in it's own repo under https://github.com/opentracing-contrib/

@rnburn may take this on soon, but we always welcome community contributions if you would like to take a shot at it @calder.

[billowqiu]

why java,py,go in this repo? but c++ in other repo?

[rnburn]

All the languages are being moved into separate repos: python, go, java