Add offer_created/offer_removed/offer_updated to effects

[johansten]

The only trade related effect I've seen in the ledger is 'trade'. None of the others show up.

[johansten]

Just to clarify why this is important,

The only reasonable way I've found to create an activity history for an account is to stream effects, then load the corresponding operation, and transaction. With no effect output for these, it makes it really hard to make a trading application, if you want to get some sort of trade history shown.

For a generic wallet, what is available is fine, because you most probably don't care about the offers themselves, just the actual trades.