Add extra methods to subscription

* #billing_day_of_month
* #billing_period_start_date
* #billing_period_end_date
Update NEWS.md with info about Subscription dates
Add specs to test customer and subscription retrieval by custom integer id
implement id normalization for custom ids, which can be provided upon customer and subscription creation.
fixes issue where retrieval of entity is not possible if it was stored with an integer as id.
Merge branch 'fix-customIntegerIdRegistry' into custom