Add a new method to fetch all events belonging to a charge

[som-m]

Summary

• [x] Add a new method to fetch all events belonging to a charge (Omise doc)
• [x] Add test

QA

You should be able to fetch a list of events belonging to a given charge.

1. Retrieve a charge charge = Omise::Charge.retrieve("chrg_test_5mozta4qq7c8rt2doub")

2. List all events of the charge charge.events