Example net/http server and client with OpenCensus

This example uses:

net/http to create a server and client.
The OpenCensus net/http plugin to instrument the server and client.
Debugging exporters to print stats and traces to stdout.
```
$ go get go.opencensus.io/examples/http/...
```

First, run the server:

$ go run $(go env GOPATH)/src/go.opencensus.io/examples/http/helloworld_server/main.go

Then, run the client:

$ go run $(go env GOPATH)/src/go.opencensus.io/examples/http/helloworld_client/main.go

You will see traces and stats exported on the stdout. You can use one of the exporters to upload collected data to the backend of your choice.

You can also see the z-pages provided from the server: