ruby : extend API (#2551)

ruby : extend API (#2551)

* Handle objs in Ruby code

* Add task to make Makefile

* Share commont constance in test suites

* Add model-related APIs

* Add Whisper::Model class

* Add tests for Whisper::Model

* Add missing LDFLAG -lstdc++

* Add tests for Whisper.log_set

* Add Whisper.set_log

* Define log level

* Add document on logging

* Add license section to README

* Add document on Whisper::Model

* Fix examples in README

* Add test for Model with GC

* Make dependency on Makefile more accurate

* Fix bug about Whisper::Model and GC