No logging after upgrading to Savon 2

Global configuration have been disabled in Savon 2 so it is not possible to set logging options in rconomic. So i would like to make a patch to get the loggin functionality back. The big question is how the interface for a economic configuration should be?

I like the way Faraday (https://github.com/lostisland/faraday) does it. This would mean that you configure a rconomic session as follows:

```ruby
session = Economic::Session.new(aggr_number, usr, pswd) do |config|
  config.log_level :info
  config.logger      Rails.logger
  config.log            true
end
```
The big win of this is that it is backwards compatible. Because if no block is passed (as before) it would just mean that no configuration is needed.