Added logging to configuration

Usage :

LicenceToBill.configuration do |config|
  config.business_key = 'xx'
  config.agent_key = 'xxx'
  config.logging[:logger] = ::Logger.new(STDOUT)
  # Theses are HTTParty defaults
  config.logging[:log_level] = :info # Anything allowed
  config.logging[:log_format] = :apache # use :curl for a far more verbose output
end
Bug with HTTP 204 (No content) responses : params is nil