Use a Request object when querying the SureTax API
  - Build an API query using Request
  - Ask the Request instance whether it is valid
Extract validation methods to module
Move Validatable module into Concerns namespace

This makes it clearer that Validatable is to be used as a mixin.
Add Request validations
- Validation method names reflect the specific business entities they are supposed to validate.
- Use a uniform style with "is_a_valid_xxxxx?".
- Phone number expectations are in shared examples
- Validate required and optional fields
Use Array#include? instead of regex
Add synopsis to readme
Refactor validation matchers for maintainability
Add RequestItem validations

Added validations for the last remaining fields in RequestItem
Remove is_ and is_a_ from validation method names
Use boolean conversion instead of checking match value
Use boolean logic instead of short-circuiting where possible
Move comment before method definition
Merge pull request #17 from allolex/add_request_objects

Add Request objects