feat(really-core): Implement executeOnGet

- Implement HiddenFields class that represent list of hidden fields and has hide funcation
- Implement OnGet Interface
- Implement executeOnGet function on Model Class
- Implement test cases for executeOnGet
Merge pull request #3 from SalmaKhater/feat-execute-on-get

feat(really-core): Implement executeOnGet
Initil implementation, lack tests, and concrete error codes
Fixes Error messages to be easily converted to json
Finalizes implementation and adds testcases
Fixes the rest json writers to be more idiomatic
Review fixes