reader: by default, archive failed messages to disk

[jehiah]

This is a follow up to #134 and changes the default behaviour of Reader.give_up() to be less surprising by writing messages that are "given up on" to disk instead of dropping them completely.

[jehiah]

RFR @mreiferson. I'd love any naming comments you have, and if you can think of an obvious way to disable this (an additional parameter of archive_abandoned_messages=True ?)

[mreiferson]

@jehiah should we also bump max_attempts to 25?

[mreiferson]

Do we need to provide a param to disable archiving when we already encourage users to subclass and override the method?

[mreiferson]

what're we doing on this one?