List and close existing tabs

[davidcockbill]

Provide access to the HTTP closing of targets.

The interface defines the following:

**GET /json or /json/list**

A list of all available websocket targets.

[ {
  "description": "",
  "devtoolsFrontendUrl": "/devtools/inspector.html?ws=localhost:9222/devtools/page/DAB7FB6187B554E10B0BD18821265734",
  "id": "DAB7FB6187B554E10B0BD18821265734",
  "title": "Yahoo",
  "type": "page",
  "url": "https://www.yahoo.com/",
  "webSocketDebuggerUrl": "ws://localhost:9222/devtools/page/DAB7FB6187B554E10B0BD18821265734"
} ]

and

**GET /json/close/{targetId}**

Closes the target page identified by targetId.

For valid targets, the response is 200: "Target is closing". If the target is invalid, the response is 404: "No such target id: {targetId}"

The former is already implemented (just not accessible via the client).