New: PR#5 Add offline xAPI statement queue (fixes #5)

[joe-replin]

New

• Added comprehensive offline statement queue system for reliable xAPI statement delivery in high-traffic and poor connectivity scenarios
• Created OfflineQueueModel.js to manage statement batching, queuing, and intelligent retry logic
• Created CreatedStatementModel.js for queue creation tracking statements
• Created ReleasedStatementModel.js for successful queue flush tracking statements
• Integrated offline queue functionality into StatementModel.js:
  • Automatic queuing of failed statements after exhausting retry attempts
  • Configurable retry delays with exponential backoff (up to 5 attempts)
  • Intelligent batch flushing when queue threshold is reached
  • Opportunistic queue flushing after successful statement sends
  • Force queue flushing on course completion and window unload
• Added queue event handlers (onQueueCreate, onQueueRelease) and send methods (sendCreated, sendReleased)
• Added comprehensive offline queue configuration in example.json:
  • _isEnabled: Enable/disable queue functionality (default: true)
  • _retryDelays: Configurable retry delays array with exponential backoff [0, 2000, 5000, 10000, 20000]ms
  • _requestTimeout: Maximum timeout for initial statement sends (20000ms recommended)
  • _queueFlushTimeout: Timeout for queued statement sends during flush (10000ms)
  • _queueThreshold: Minimum statements required before automatic batch flush (5 statements)
• Added complete "Offline Queue Configuration" documentation section in README with:
  • Detailed property descriptions and behavior explanations
  • Recommended settings for high-traffic deployments (200k+ users)
  • Critical vs normal statement handling documentation
• Added created and released verbs to xAPI verbs table in README
• Replaced _.extend() with Object.assign() in ReleasedStatementModel.js for ES6 compliance

Testing

1. Install or ensure xAPI LRS connection is configured
2. Enable offline queue in course/config.json:

   "_xapi": {
     "_tracking": {
       "_offlineQueue": {
         "_isEnabled": true,
         "_retryDelays": [0, 2000, 5000, 10000, 20000],
         "_requestTimeout": 20000,
         "_queueFlushTimeout": 10000,
         "_queueThreshold": 5
       }
     }
   }