userManager: Manage groups of users recording in XML files

This class can be used to manage groups of user storing user and group records in XML files.

It create a XML file with user and group information and perform several other types of operations to manipulate users and groups information, like:

- Add an user group
- Add an user to a group
- Change the name of a group
- Remove a group
- Add an user
- Change the password of an user
- Remove an user
- Verify whether a password is correct for a given user
- Get the access of an user