v0.6.1

Only small changes and bug fixes + ReceiverBuilder and queue saving strategy.

• Introduced a new invariant: all items have to be read and used by the end of every
  transaction. I could not verify if this invariant always holds. Anyway, there is an
  assertion in the code to avoid the worse. If you find such a situation, please fill an
  issue.
• Dropping the Receiver forced the state to be saved, not the initial_state (the
  state at the begining of the current transaction). Now, Drop calls Receiver::save
  so that the behavior will be always consistent.
• We have a backup strategy for saving the queue! It invlves no asyc stuff, so it will
  only be triggered at the end of a transction. The current criterion is: save at every
  250 items read or every 350ms, whichever comes first. This should dimiinish greatly
  the necessity for external control of the save mechanism.
• Created a ReceiverBuilder to allow people to costumize the way the queue is saved.
  This includes altering the above defaults or disabling queue saving altogther.