Use defaults in Suretax.configuration

- Allow arguments passed in to initializers to override defaults.
- Include configuration example in the Readme
- Use default client_number
- Use default validation_key
Merge pull request #26 from Curatur/configuration_improvements

Configuration improvements
refactor validations for better error diagnosis, remove request configuration parameter
Merge pull request #27 from bqsoft/refactor_validations_and_remove_configuration_parameter

refactor validations for better error diagnosis, remove request configuration parameter