refactor for object model support

[benjodo]

Here's a summary of the changes we've made to implement a Ruby object model for the Pylon API client:

1. Created Model Structure:
   - Implemented Models::Base class with attribute access through method_missing
   - Added specific model classes for all API resources (User, Issue, Account, etc.)
   - Created a Collection class that implements Enumerable for lists of resources
2. Updated Client Implementation:
   - Modified all API methods to return model objects instead of raw hashes
   - Added model_class and collection parameters to HTTP request methods
   - Updated handler_response to properly wrap responses in model objects
   - Maintained backward compatibility with the previous hash-based approach
3. Added Documentation and Examples:
   - Updated the README with examples using the new object model
   - Created a migration guide to help users transition to the object model
   - Added API validation script to verify model compatibility with the actual API
4. Updated Tests:
   - Modified all tests to work with the new object model
   - Added tests for the model classes themselves
   - All tests are passing with 84.69% line coverage
5. Updated Version and Changelog:
   - Bumped version to 1.1.0 (as a compatible feature addition)
   - Updated CHANGELOG.md with detailed information about the new object model

The new object model provides a much more intuitive Ruby interface while maintaining backwards
compatibility. Users can now use methods like issue.title instead of hash access issue["title"], and
collections are directly enumerable with methods like each, map, etc.

To validate against the current API documentation, you can use the api_validation.rb script we created,
which will compare our model structures with the actual API responses.