Provide default values for Suretax API endpoints.

No, we don't need to define ```def base_url=``` because the class declares :base_url as an ```attr_accessor```.  The difference between this instance variable and the two that follows is that I need to make sure that we only accept known versions of the request/cancel API so I provided concrete setters that do some value checking for them.  Since we don't know how Suretax will define the urls for different customers we cannot do the same thing with base_url.

With this setup what's stated in the README works:
```ruby
ENV['SURETAX_BASE_URL'] = 'https://mysubdomain.taxes.com'

Suretax.configure do |c|
  c.validation_key = ENV['SURETAX_VALIDATION_KEY']
  c.client_number = ENV['SURETAX_CLIENT_NUMBER']
  c.base_url = ENV['SURETAX_BASE_URL']
end

Suretax.configuration.base_url
#=> 'https://mysubdomain.taxes.com'
```