allow group by expression

[anjmao]

#375

[coveralls]

Coverage increased (+0.02%) to 93.417% when pulling a791257 on group-by-exp into bc6efe3 on master.

[rajhim]

what if group is like this ? Iam not able to implement it. Can I have suggestion Please?
i need to implement like this
-categoryName
-services.

> [
>   {
>     "categoryName": "FaceWash",
>     "services": [
>       {
>         "serviceName": "Face",
>         "price": 250
>       },
>       {
>         "serviceName": "Face & Neck",
>         "price": 300
>       }
>     ],
>     "categoryId": "faceWashId"
>   },
>   {
>     "categoryName": "BLEACH ",
>     "services": [
>       {
>         "serviceName": "Jawline",
>         "price": 150
>       },
>       {
>         "serviceName": "Face",
>         "price": 300
>       },
>       {
>         "serviceName": " Face, Neck & Blouse Line ",
>         "price": 600
>       },
>       {
>         "serviceName": "Stomach",
>         "price": 450
>       }
>     ],
>     "categoryId": "BODYPOLISHBLEACH"
>   },
>   {
>     "categoryName": "BODY POLISH & WRAPS",
>     "services": [
>       {
>         "serviceName": " Instant Body Polish ",
>         "price": 2000
>       },
>       {
>         "serviceName": "Back Polish",
>         "price": 1250
>       },
>       {
>         "serviceName": " Body Polish + Massage & Wrap ",
>         "price": 3000
>       }
>     ],
>     "categoryId": "BODYPOLISH"
>   }
> ]