Add more information about build in json output stream.

Summary:
The json output stream now contains the command being run, the command's exit
status and the build's result (success, fail).

Example of json output:

  {
    "id": 0,
    "cmds": [
      { "id": 0, "cmd": "/usr/bin/c++ main.cpp -o main.o"},
      { "id": 0, "status": "SUCCEEDED" },
      { "id": 1, "cmd": "/usr/bin/c++ main.o -o falcon"},
      { "id": 1, "status": "SUCCEEDED" }
    ],
    "result": "SUCCEEDED"
  }

Test Plan: Start the daemon, and trigger a build with the client.

Reviewers: NicolasDP

Reviewed By: NicolasDP

Differential Revision: http://91.121.199.76/D33