Implement the webactions at the proxy runtime level

[sciabarracom]

Currently the go proxy for runtimes does not answer in port 8080 at the root level (the answer is empty). It answer to /run and returns a json.

The output for webactions is defined by this spec

In order to improve testability and also support knative using our runtimes, we should implement input and interpret the output of a web action directly in the proxy.

So if we invoke the proxy at the root: (http://localhost:8080/) it should work like /run but we should pass the parameters __ow_* as defined in the spec and then interpret the output according the webactions spec. For example if an action returns:

{ 
  "body": "<h1>hello</h1>"
}

it should NOT return the json but actually <h1>hello</h1>, with an appropriate content type (default text/html).

We should also implement the other fields. For example invoking http://localhost:8080/ for this aswer

{ 
  "statusCode": 307,
  "headers": { "Location": "https://www.google.com" } 
} 

will return an empty body with status coden 307 and the header Location to implement a redirect.

Finally we should support images:

the answer:

 { 'Content-Type': 'image/png' },
    statusCode: 200,
    body: "xxxxx" 
}

when the content type starts with image/ should decode the body as base64 encoded.

OpenWhisk also allow for setting the content tye extensions, so we should add the ability to use /index.html to returnthe body with default content-type text/html, while index.json will use content-type: application/json, index.txt returns text/plain, index.png and index.jpg returns image/png and image/jpeg.