Add test cases and fix #5

Author: marijnvdwerf

composer.json

@@ -24,6 +24,11 @@
       "marijnvdwerf\\palette\\": "src/"
     }
   },
+  "autoload-dev": {
+    "psr-4": {
+      "marijnvdwerf\\palette\\Tests\\": "tests/"
+    }
+  },
   "require": {
     "php": ">=5.3",
     "intervention/image": "~2.2"
@@ -34,6 +39,7 @@
   },
   "require-dev": {
     "devster/ubench": "^1.1",
+    "phpunit/phpunit": "^6.1",
     "symfony/debug": "~2.6",
     "symfony/var-dumper": "~2.6"
   }