User config - Part 1

This is part 1 of the user config implementation: 

- Introduces a new Python configuration module for the user which allows him/her to change his/her avatar, real name and password.
- Introduces a new panel applet called user applet which allows common session functions (logout, switch user, lock screen) and shows info about the logged in user.

Part 2 will replace the GNOME "User Accounts" tool and introduce a system python module called "Users and Groups".