Feature: introduce un.Users.Statistics method as well

[hbagdi]

This is related to #9
There will two ways of doing the same thing.
Once via as a method on user struct and another directly on the UserService via a username.

Both will need a new opt struct to specify resolution and quantity.

Parameters
param Description
username The user’s username. Required.
resolution The frequency of the stats. (Optional; default: “days”)
quantity The amount of for each stat. (Optional; default: 30)

Currently, the only resolution param supported is “days”. The quantity param can be any number between 1 and 30.