Adding short license text in each file

[Saluki]

Since the project has moved to the GNU General Public License v3, all developed files should start with the following license text:

Barmate.
Modern and intuitive POS web application written with the Laravel framework.

Copyright (C) 2015  Corentin Badot-Bertrand  (http://badot.corentin.me)

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.