Refactored to use active-record style api

Updated to match BillForward's style guidelines.

From Ian Saunders:
> Type Creation is from a static method. For example: https://app-sandbox.billforward.net/api/#/method/accounts/POST?path=%2Faccounts&api=Java
Updating models is from instance methods. For example: https://app-sandbox.billforward.net/api/#/method/accounts/PUT?path=%2Faccounts&api=Java
Getting objects is from static methods. For example: https://app-sandbox.billforward.net/api/#/method/accounts/GET?path=%2Faccounts%2F%7Baccount-ID%7D&api=Java

For example:
```
@subscriptions = Aptible::BillForward::Subscription.all
@subscription = Aptible::BillForward::Subscription.find(subscription_id)
@subscription.create_usage_session(uom: params[:uom])
```