Transactions - 4

[wesleylin]

• travis.yml added new env with the api key (wesleys) for testing purposes
• added new client.py that will add the key for all request classes and create a singleton to call
• client fetchs key either from Client('xxxx_key_here_xxxx') or as an environment variable NESSIE_API_KEY=xxxx_key_here_xxxx
• added new customer tests
• added new atm tests
• fixed customer class slightly with exceptions