Add client number and default post/cancel paths to configuration

- Include configuration example in the Readme
Allow per-connection configuration

The default configuration is set via Suretax.configure, but you can override the defaults set there by creating a new Suretax::Configuration instance and passing it as the argument to Suretax::Connection.new.
Remove "default" from configuration accessors
Use configuration defaults in requests

- Use default client_number
- Use default validation_key
- Allow defaults to be overridden by passing a Configuration object into Request
Update readme with correct configuration attributes
Use rubyish do…end and curly braces in specs
Avoid passing configuration in via initializer

Passing in configuration via the initializer is ugly, so we're going to refer to the singleton directly in the class code. Suretax.configuration is inside a wrapper method to keep things tidy.