Adds support for each with concurrency

[chebyte]

• Adds each_with_concurrency method
• Updates Readme

[seivan]

@scalp42 @chebyte Sorry for waking this up, but I'm doing some research on how to use Redis with a distributed lock for queues and I was wondering, how does this PR handle removing from the backup queue? In a perfect world order wouldn't matter and if you process 5 items, then 5 will be popped. But in scenarios where exceptions are thrown you might pop the wrong items.

[seivan]

I see that this commit is missing https://github.com/soveran/ost/pull/8/files#diff-e007f35caa5af71683eeaa305ba27a2cR38 that would make this actually work. But then you'd remove specific items instead of popping the stack.