Give separate events within an ecommerce transaction different transaction IDs

[fblundun]

This is a partial revision of #25.

The events will have the same order_id, so there is no need for them to have the same randomly generated tid.

[alexanderdean]

Good catch Fred - no reason for these events to have the same tid. tid is meant to be a nonce, different across different events!

[alexanderdean]

Done in 0.4.0, closing