Update ImageLoader.php

ColorThief\Image\ImageLoader::load to accept remote URL
Update ColorThiefTest.php

Added remote image test
Update ColorThiefTest.php

Fixed undefined variable. Whoops, should have tested that first.
Fix expected colour for remote image test
Merge pull request #13 from rewmike/master
Improve tests.
Remove duplicated tests.
Add testLoad404Url()
Fix style.
Merge branch 'issue-13'