Add a function to return the UserService object

Problem/Motivation

The underlying Jira library that the 4.x version of jira_rest depends on providers a UserService class, which can be used for CRUD operations on user accounts. However, jira_rest does not expose that object via the API.

Steps to reproduce

1) Try to perform user account operations on Jira.
2) UserService API is not exposed.

Proposed resolution

Add a function to instantiate the UserService and return it to the calling function.

API changes

A new `getUserService()` function has been added to the JiraRestWrapperService service.

I've included a patch below, could you let me know your thoughts?