Add 'put-unique' command.

This command can be used to solve many of the de-duplication issues
by not allowing duplicate messages in the queue to begin with. It has
no impact on the performance on the regular put command as it is
implmented as a parrallel command. Instead of using the job number
or name to try and find wither a message exists already, this command
compares the entire message body to determine uniqueness.

This code is in production using a 1.4.6 version of beanstalkd. This
is a quick forward port, which has not been thoroughly tested, but the
porting was straightforward, so there probably won't be any suprises.