method for getting ConnectionPool utilization

Recently I can in a situation where too many connections where opened, causing the 'pending' connections to stack up more and more. For debugging this, it would have been helpful to have a call printing out the current utilization of the `ConnectionPool`, e.g.:

``` ruby
def info
  {
    available: @available.count,
    reserved: @reserved.count,
    pending: @pending.count
  }
end
```
