Add "Customer" related functionaility

[sumitsouda]

jeremymarc, you may know that the "Accounts" i.e. creating account,add bank account to the account and etc. things are "Deprecated" by the Balanced Payment. Instead of "Account" they are recommending to use "Customer". As I discussed with the Help Support of Balanced Payment, they will not be included "Account" in the next major API version. So there is a need to add "Customer" funcionality.

[jeremymarc]

I didn't know about that... I will have to check, or if you have the time, feel free to do a PR.