queue: starting to migrate the queue to an interface

The queue itself is already migrated. Time to migrate the Handler :-)

Related to #305.
queue: prepare QFactory to return a handler
queue: generalize handler

Now a Handler is just a type that runs a function until Stop is called
:-)
queue: deleting some code

Related to #305.
queue: change handler name to executor, introduce Handler interface

Related to #305.
queue: move Handler interface

Also renaming file and test suite :-)
queue: added compile-time check for interface implementation
queue: improve wording in Queue docs
queue: fix docs for the package
queue: added compile-time test to make sure that beanstalkQ satisfies Q
queue: fix QFactory definition
queue: readd Delete method to the queue

This method will just mark the message to be delete. The handler will
later delete it.
queue: don't call add for already running executors
queue: change QFactory.Handler sgnature

Just the message should be enough.
queue: added beanstalkFactory

This factory implements the Get method, that will return a beanstalkQ
that operates on the given queu name; and the Handler method, that
returns a handler that will call f whenever a new message arrives at one
of the watched queues.
queue: make beanstalk handler delete the message when user call Delete()
queue: beanstalk handler release the message when user don't delete it

Related to #305.
queue: improve docs for Handler
queue: register beanstalkFactory as one of available factories
queue: implement Factory function

This function reads "queue" from the configuration file, and returns the
instance of the QFactory to be used.

Related to #305.