Add get phone pricing functionality

[djbloc]

The get-phone-pricing API functionality does not seem to be included. This API returns either sms or voice pricing for the supplied phone number.

Example/details at the bottom of the following page https://docs.nexmo.com/index.php/developer-api/account-pricing

GET /account/get-phone-pricing/outbound/{product}/{api_key}/{api_secret}/{phone}