Username/AccessToken provided via Environment variables

Username and AccessToken are imported using environment variables. If they're missing then a useful error is raised. README updated to explain how to set this up, doc added in test_bitly_api.py to explain how to run the test.

One test changed in test_bitly_api as the 'test1' key is a valid bitly key, maybe this entered the keyspace? I've added a longer random string, I don't know if this keeps the same meaning to the test (this needs validation).