Returning an array of rentals instead of just one

Based on @jensfrandsen request:
> When doing a getRentalStatus the {"message":"You have no active rental"} message is returned. That's not the easiest way for the app to decipher that. Right now I check whether my expected data has not been set, but that not a very pretty way to do it either. A better way could be to change the API so that it returns an array of rentals.
> So change
>   ```json
>  { 
>    "lock_id": 1,
>    "start_time": "2014-08-18T18:30:14.197Z",
>    "lock_identifier":"Bennyburro"
> }
> ```
> to
> ```json
>  {
>   "rentals": 
>   [ {
>      "lock_id": 1,
>      "start_time": "2014-08-18T18:30:14.197Z",
>      "lock_identifier":"Bennyburro"
>   } ] 
> }
> ```
> in case of no active rentals:
> ```json
> {"rentals": [ ] }
> ```
> Its easy to check for a zero length array and this would also support if we were ever to go for multiple concurrent rentals.