Added QueryCounter helper class for specs.

This is a simple class for helping us do query counts in specs.  There is also a matcher to allow spec writers to limit the number of queries if desired.

We could potentially use this to count all queries in all specs with something like the following in spec_helper.rb

```ruby
  config.around do |example|
    puts "Queries: #{QueryCounter.count(&example)}"
  end
```

@jrafanie Please review.
That doesn't seem like a bad idea, but it does change what this class does and the name is no longer accurate. ...perhaps it shouldn't be called QueryCounter anymore.  Perhaps QueryDiagnostics?  Seems wordy.