Add copyright of library authors

Author: Alexander I.Grafov

reader.go

@@ -5,6 +5,7 @@ package m3u8
  This file defines functions related to playlist parsing.
 
  Copyleft 2013-2015 Alexander I.Grafov aka Axel <grafov@gmail.com>
+ Copyleft 2013-2015 library authors (see AUTHORS file).
 
  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

reader_test.go

@@ -2,6 +2,7 @@
  Playlist parsing tests.
 
  Copyleft 2013-2015 Alexander I.Grafov aka Axel <grafov@gmail.com>
+ Copyleft 2013-2015 library authors (see AUTHORS file).
 
  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

structure.go

@@ -5,6 +5,7 @@ package m3u8
  This file defines data structures related to package.
 
  Copyleft 2013-2015 Alexander I.Grafov aka Axel <grafov@gmail.com>
+ Copyleft 2013-2015 library authors (see AUTHORS file).
 
  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

structure_test.go

@@ -2,6 +2,7 @@
  Playlist structures tests.
 
  Copyleft 2013-2015 Alexander I.Grafov aka Axel <grafov@gmail.com>
+ Copyleft 2013-2015 library authors (see AUTHORS file).
 
  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

writer.go

@@ -5,6 +5,7 @@ package m3u8
  This file defines functions related to playlist generation.
 
  Copyleft 2013-2015 Alexander I.Grafov aka Axel <grafov@gmail.com>
+ Copyleft 2013-2015 library authors (see AUTHORS file).
 
  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

writer_test.go

@@ -2,6 +2,7 @@
  Package m3u8. Playlist generation tests.
 
  Copyleft 2013-2015 Alexander I.Grafov aka Axel <grafov@gmail.com>
+ Copyleft 2013-2015 library authors (see AUTHORS file).
 
  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