Made long polling (via BLPOP) the default funcionality

This is in regard to #150.

The original default functionality, short polling (via LPOP), can accessed by setting the `SHORT_POLLING` environment variable.

Note: There is a test in `WorkerTest` that I marked as incomplete because, although it passes, it does not actually test blocking list pops. But I didn't delete it because I wasn't sure what it was for. This will need to be addressed.