BillingApi calls Super without a userAgent

The billingAPI should specify a userAgent when calling super.
... around line 22 of baseApi.py
```
class BaseAPI(object):
  def __init__(self, AccessToken, apiServerAndVersion, userAgent, timeout=10, verbose=False):
```
... around line 12 of billingApi.py
```
class BillingAPI(BaseAPI):
        super(BillingAPI, self).__init__(AccessToken, apiServerAndVersion)
```