Add attendance tracking and history.

1. REGISTERED_NO_SHOW is used to mark a user as absent.
Absent users will not see the event in their impact timeline.

2. event.organizerProcessedCompletion is new and meant to be used to track whether a completed event has been processed by the organizer - meaning has the organizer taken attendance. To check if an event is completed check the status field to see if it is "COMPLETED".

2. Users now have an event history for the last 10 events
user.eventAttendanceHistory
user.eventAttendanceHistoryPct

````json
{
  "eventAttendanceHistoryPct":80.0,
  "eventAttendanceHistory":[
    {
      "event":{
        "key":"agprZXgtbGF0ZXN0cgwLEgVFdmVudBiPAQw"
      },
      "eventStartTime":1376780400000,
      "attended":true
    },
    {
      "event":{
        "key":"agprZXgtbGF0ZXN0cgwLEgVFdmVudBiQAQw"
      },
      "eventStartTime":1376175600000,
      "attended":true
    },
    {
      "event":{
        "key":"agprZXgtbGF0ZXN0cgwLEgVFdmVudBiSAQw"
      },
      "eventStartTime":1374966000000,
      "attended":true
    },
    {
      "event":{
        "key":"agprZXgtbGF0ZXN0cgwLEgVFdmVudBiTAQw"
      },
      "eventStartTime":1374706800000,
      "attended":false
    },
    {
      "event":{
        "key":"agprZXgtbGF0ZXN0cgwLEgVFdmVudBiUAQw"
      },
      "eventStartTime":1374102000000,
      "attended":true
    }
  ],
}
````