Improving counting of seen transactions in notification message

[cortinico]

Fixing the count of transactions that is shown on the notification.
Apparently this code:

if (transaction.getStatus() == HttpTransaction.Status.Requested) {
            transactionCount++;
}

was causing a miscount of the seen HTTP calls. If a call is already .Completed by the time it gets reported to Chucker, the transactionCount variable won't be updated.
I'm replacing the simple counter with an HashSet of transaction IDs. This will allow us to precisely count how many HTTP Request/Response we saw and report it in the notification number.

Fixes #84