Add "use" statement to load correct class

This prevents errors of the type "Argument 1 passed to Gregwar\Image\Adapter\GD::merge() must be an instance of Gregwar\Image\Adapter\Image, instance of Gregwar\Image\Image given" caused by Gregwar\Image\Adapter\GD looking in the same folder for the Image class in all methods with an "Image" typehint
Merge pull request #38 from jfoucher/master

Add "use" statement to load correct class