add ability to set read_timout fot Net:HTTP - resolves #43

I can now set the read_timeout attribute using the following snippet. The attribute read_timeout [defaults to 60 seconds](http://www.ruby-doc.org/stdlib-2.1.1/libdoc/net/http/rdoc/Net/HTTP.html#method-i-read_timeout-3D).

```
class AppProxy < Rack::Proxy
  def rewrite_env(env)
    env["http.read_timeout"]=300
    env
  end
end
```