Add a `before_reserve` hook

This is the stab at a `before_reserve` hook as suggested by @defunkt. I feel like this might be better one level up in `Worker` but this follows the pattern that this code base already is using.

This would allow someone to do...
```
Resque.before_reserve do |queue|
  raise Resque::Job::DontReserve if queue == "high"
end
```

cc @ymendel @tnm