Simplify message acknowledgement

add `ack()` and `nack()` methods to the message
object handled by the `subscribe` callback.
Simplify unsubscription

instead of returning an id from `client.subscribe()`, return
a literal with an `unsubscribe()` method that can be called directly.
simplify transaction

the `begin()` method returns a JavaScript object with `commit()` and
`rollback()` methods to complete the transaction.
fix WebWorker example
tag version 2.1.0
prepare next release