Param binding

Hello, 

It is sometimes bothering to escape parameters in query properly. So, I wrote a patch for that.

By this patch, we become to able to write queries with parameters like this:

```ruby
describe server(:db) do
  describe mysql_query('SELECT User, Host from mysql.user WHERE User=? AND Host=?', 'app', '%') do
    it '...' do
      # ...
    end
  end
end
```

Could you consider this?
Thank you.