Add RecordChanged event to records

[oriash93]

fix #36

[oriash93]

the event can be used as follows:

IDeepStreamRecord record = await records.GetRecordAsync(someRecordId);
record.RecordChanged += OnRecordChanged;

where OnRecordChanged is:

private void OnRecordChanged(dynamic recordData, EventArgs e)
{
    \\ Do something with recordData
}