Refactoring : interfaces

Hi Jurians,

I realized my previous pull request would be just impossible to review so I decided to split it up. Once again, I really think it would be a better idea to have SlmQueue / SlmQueueSqs / SlmQueueBeanstalk (otherwise it will be really a pain if someone add another one as people will need to download all libraries). You can put me as a maintainer for the three modules, I can help you on this one, no problem!

Anyway, this PR is mostly interfaces and AbstractClasses. Tell me what you think, so that we can review it in smaller chunks, if it's okey I can write tests, we can merge and then I can work on Beanstalk first :).