Allow different configs per client

[danielcooper]

The current implementation sets the configuration at a class level - this allows an optional instance level configuration so you can do things like:

# Default client
TravelTime.configure do |config|
  config.application_id = ENV.fetch('TRAVEL_TIME_APP_ID', '')
  config.api_key = ENV.fetch('TRAVEL_TIME_API_KEY', '')
  config.raise_on_failure = true
end

# Client for batch operations
TRAVEL_TIME_BATCH_CLIENT = TravelTime::Client.new.configure do |config|
  config.application_id = ENV.fetch('TRAVEL_TIME_BATCH_APP_ID', '')
  config.api_key = ENV.fetch('TRAVEL_TIME_BATCH_API_KEY', '')
  config.raise_on_failure = true
end