Unit Tests for SQS - Push support for SQS utilizing SNS - Overhaul of SubscribeCommand - Addition of UnsubscribeCommand - Move setContainer to Job base class and more

The branch probably isn't named appropriately, but the title of this PR pretty much sums up what would be included.  

This PR has grown out of a few other proposals/PRs from myself and @SammyK (#3759, #3778, #3784, #3789) so I'd like to get some more eyes on the additions/deletions/changes that are included in the 23 commits included in the branch (I know I need to squash them down to 1)..

First, a little background on this.  I started out by writing unit tests for SqsQueue and SqsJob since there were none and also made a couple other small changes. @taylorotwell wanted me to submit just 1 PR to cover those few things. Then, @SammyK proposed (and implemented) adding push support for SQS, so I wanted to get that included since there were going to be merge conflicts (so I borrowed his contribution and took care of the conflicts). I was trying to ensure @SammyK received the official and much deserved credit for the initial addition of SQS push support by making him a contributor to my fork of laravel/framework (so he can push his files in and be recorded as a contributor).  But, I haven't heard back from him since 6 days ago and I need to move on (this was a huge tangent from working on my own project that's built with Laravel).  

I'd like to make sure this is all worthy of being included in the core of the framework and make sure any issues that people see are addressed so I can get everything ready to be merged.  

It's hard to tell if I broke anything with my SubscribeCommand refactor since there weren't any unit tests and I was only using it for testing the IronMQ and SQS/SNS push queue functionality using the default queue config.  I didn't really like how it was using IronMQ's updateQueue function (rather than the addSubscriber) for subscribing (along with all the functions for gathering meta data for the queue) since that seems better suited for a new command, something like `php artisan queue:update -whatever` (via a new UpdateCommand).  However, I did make sure to call updateQueue after call to addSubscriber just in case it actually is needed (minus the subscribers part of the options parameter).  I also didn't like the checks for `if($queue instanceof IronMQ)` since it seems like (and I think I've shown) it can be better handled through polymorphism.  Additionally, I felt like the setContainer functions that were in every derived class of Job should be removed and just defined in the base class.  

There are also a bunch of unit tests I wrote, so I'd love to get some feedback from a phpunit expert.  Anyway, there are a lot of other changes to discuss and get feedback on, so please have a look.  Thanks for taking the time.