[ENHANCEMENT] Add "retryDoer/hystrixDoer" implementation

[MichaHoffmann]

When comparing https://github.com/gojek/heimdall/blob/master/httpclient/client.go and https://github.com/gojek/heimdall/blob/master/hystrix/hystrix_client.go it seems that they only differ fundamentally in their respective Do methods. One could factor that part out into seperate Doer implementations and only have one httpclient implementation that delegates to a configurable Doer.

I would be willing to file a PR if that is acceptable.

[rShetty]

Sure @MichaHoffmann you can work on this