Bug 1035668 — Add BeepSend SMS Gateway.

https://bugzilla.mozilla.org/show_bug.cgi?id=1035668
> We will not have dozen of them 

we're going to have 3 now and I am pretty sure we will gain more.

> so I supose it will be better documented if we keep it separate.

From our IRC conversation, Remy was thinking about making sure we are still about to document it using convict, so we need to be carefull about how it's structured

My latest proposal:

```json
smsGateways = { nexmo: { options }, 
                              beepsend: { options } 
}
```