Add diagonal line graphic (^GD)

[LagTag]

This is a part of Issue #30. It adds the diagonal line graphic command (^GD), which has attributes of
box_height, box_width, line_thickness, color, orientation.

label = Zebra::Zpl::Label.new(
    :width         => 800,
    :length        => 800,
    :print_speed   => 6,
    :print_density => 5,
    :copies        => 1
)
label_diagonal_test1 = Zebra::Zpl::Diagonal.new :position => [400, 400], :box_height => 100, :box_width => 50,:line_thickness => 5, :color => "B"
label_diagonal_test2 = Zebra::Zpl::Diagonal.new :position => [400, 400], :box_height => 100, :box_width => 50,:line_thickness => 5, :color => "B", :orientation => "L"

label << label_diagonal_test1
label << label_diagonal_test2
label.dump_contents
>>^XA^LL800^LH0,0^LS10^PW800^PR6^FO400,400^GD50,100,5,B,^FS^FO400,400^GD50,100,5,B,L^FS^PQ1^XZ