Updated OSX icon file to support high resolution assets

I have updated `icon.icns` file with high resolution images (up to 1024x1024 pixels).

![hydrogen-crop](https://cloud.githubusercontent.com/assets/1619113/2636661/13c3c648-be94-11e3-9aaa-47b17d19f8e4.png)

Here's script I used to create icon file:

    #!/usr/bin/env perl
    use strict;
    use warnings;
    
    my @sizes = qw/128 256 512/;
    my $out = "icon";
    
    mkdir("$out.iconset");
    for my $size (@sizes) {
    	system ("rsvg-convert --width=$size -o $out.iconset/icon_${size}x${size}.png icon.svg");
    	my $size2x = $size*2;
    	system ("rsvg-convert --width=$size2x -o $out.iconset/icon_${size}x${size}\@2x.png icon.svg");
    }
    
    my %e = (
    	'16x16'    => 'icon16.png',
    	'16x16@2x' => 'icon32.png',
    	'32x32'    => 'icon32.png',
    	'32x32@2x' => 'icon64.png',
    );
    
    for my $dest (keys %e) {
    	my $src = $e{$dest};
    	system('cp', $src, "$out.iconset/icon_$dest.png");
    }
    
    system("iconutil -c icns $out.iconset/");