Feature/deeply nested worker class detection

Hello, there! 

First off, thanks for the wonderful gem!  It's proven to be very helpful as I start making use of more rabbitmq services :)

While using sneakers, I ran into some issues, namely with the fact that I wasn't able to load workers that were defined in deeply nested modules, like `Foo::Bar::BizWorker`.  

This changeset augments `Sneakers::Utils` by adding the following functionality:
- `Sneakers::Util#find_const_by_classname` A new class level method that splits the class name of a deeply nested class and iteratively attempts to `const_get` until it finally reaches the desired class.  It also preserves the original funcitonality which called the top-level `Object.const_get`.

This changeset also introduces some accompanying tests, which ensure that both of these use cases perform as expected.

Thanks!