please add a overloading for delete user API

[DustFinger001]

A request param (hard_delete) already been added to user deletion api in gitlab v9.3(https://docs.gitlab.com/ce/api/users.html#user-deletion).
please add a overloading to support this feature, thx.

[gmessner]

@DustFinger001
hard_delete support has been added to deleteUser(). Changes have been pushed to master and will be available in an official release later this week.