Set default savon configuration

The client should be production ready:
 - No logging
 - If logging, then it should only be on info level
Delegate logging configuration from Session to Savon globals
Yield the endpoint on session initialization

This gives the opportunity to configure the savon client
on initialization instead of calling the setter methods
after. Example:

Economic::Session.new(123456, "user", "pwsd") do |config|
  config.log = true
  config.logger = Rails.logger
  config.log_level = :info
end