Add a non blocking LaunchAsync method to worker

Added a non blocking LaunchAsync method to worker struct as alternative to blocking Launch.

Improved Ctrl+C Handling

First Ctrl+C starts graceful shutdown (i.e. wait for running tasks to finish before shutting down). Second Ctrl+C will just abruptly end the program without waiting for tasks.

Wait For Tasks To Finish When Quitting

When there are running tasks and you call worker.Quit() method, the process will wait for all running tasks to be processed first before shutting down.

Updated Dependencies

Updated dep cli tool and all vendored dependencies.

Travis CI gometalinter bugfix

Fixed gometalinter issue on Travis CI so builds now pass successfully. Usage of native go tool vet is enforced now via gometalinter.json config file.

Return Arguments Are Now Appended To Success Callbacks

When a task has OnSuccess callback, the returned arguments are appended to the callback task instead of prepended.

MongoDB Backend Improvement

Small MongoDB Backend Improvement to allow storing task results as bson.Json.

SendGroup Error Channel Size Fix

Error channel in SendGroup could potentially have up to number of tasks * 2 size. Channel size increased to accomodate for this eventuality.

SendGroup Concurrency Fix

Fixed a bug in SendGroup pooling logic which meant all tasks would be sent synchronously irrespective of the concurrency parameter.

Release v1.0.1

When using AMQP as backend, x-expires parameter is set when declaring a queue in order to set/update task state. This ensures task state queues will eventually get cleanup up.