a message processor

it listens to notify events from the database and schedules a
message processing run. it processes then N messages at a time
until the queue is drained.

it is multi-process safe, so running multiple bodegas on different
machines should not result in missed or duplicated messages

the message type causes processing by a similarly named file in
lib/messengers which can do whatever they need to / want.

currently only sendEmail is supported and just one basic messenger
is implemented.

TODOs left:
* additional message-type-specific information with message request
* a more flexible way to connect a messenger to a given message type
* HTML email (if really wanted?)
* a way to unit test this