User access control and management

Closes #2 (closed) and #21 (closed)

Implements a user storage place, where the username and the password are stored.

Implements the MPD password command used to authenticate a user, the kill command and a custom restart command.

For db update, implements the rescan [URI] MPD call. Will need to make kara available or not depending if we have already downloaded them or not.

TODO list:

• user authentification
• password command
• kill command
• __restart command
• kara in the database can be available or not
• rescan command