Added dpi method for image conversion units

[RohanSakhale]

New Method Introduction

dpi helps to update image density supported only for imagick driver

It throws exception if called over other driver

Usage

$path = "/home/user/hello.jpg";
$image = Image::make($path);
$image->dpi(300);
$image->save();

[RohanSakhale]

@olivervogel can you please review if this is a contribution is acceptable or requires an update

[RohanSakhale]

@olivervogel been a year waiting to hear from you :-)