Adds the ability to get debug info from the backburner instance.

Adds the ability to get debug info when the backburner instance's DEBUG flag is set to true. This internally leverages the existing functionality of retrieving stacktraces from queues.

Changes include:

a new public method on the Backburner class - getDebugInfo
a new public but privileged _getDebugInfo method on the DeferredActionQueues class
a new public but privileged method on the Queue class

The intent is that this can be leveraged for additional information when identifying timer leaks within ember-qunit, which already contains some leak detection.

Sample output:

{
  "counters": {
    "begin": 59,
    "end": 49,
    "events": {
      "begin": 58,
      "end": 0
    },
    "autoruns": {
      "created": 13,
      "completed": 12
    },
    "run": 16,
    "join": 0,
    "defer": 0,
    "schedule": 17,
    "scheduleIterable": 0,
    "deferOnce": 0,
    "scheduleOnce": 10,
    "setTimeout": 1,
    "later": 13,
    "throttle": 4,
    "debounce": 60,
    "cancelTimers": 2,
    "cancel": 35,
    "loops": {
      "total": 58,
      "nested": 2
    }
  },
  "timers": [],
  "instanceStack": [
    {
      "one": [],
      "two": [],
      "three": [
        {
          "target": null,
          "stack": "Three stack"
        }
      ],
      "four": [
        {
          "target": null,
          "stack": "Four stack"
        }
      ]
    },
    {
      "one": [
        {
          "target": null,
          "stack": "One stack"
        }
      ],
      "two": [
        {
          "target": null,
          "stack": "Two stack"
        }
      ],
      "three": [],
      "four": []
    }
  ]
}

TODO:

[x] add more tests for autorun scenarios
[x] add more tests for nested runs
[x] add tests for main APIs (later, debounce, etc)
[x] add docs to README
[x] add examples of returned debug info to this PR description

BackburnerJS / backburner.js

Adds the ability to get debug info from the backburner instance. #348