Add endpoint to filter accounts by label

[zyuhel]

To better search for accounts it will be great to have an endpoint that will return accounts by partial label match, and list account for different label categories.
Also it will be good to have an endpoint to return list of all available label categories.

/labels/categories - get (return list of all label categories)
/labels/accounts/search - get (two arguments, label name, and label category), returns accounts matching the request