Add autoload information to composer.json

The commit adds the base directory of the project to the classmap way of
autoloading classes through Composer.

Resolves #5

composer.json

@@ -1,5 +1,8 @@
 {
     "require": {
       "phpunit/phpunit" : "4.8.*"
+    },
+    "autoload": {
+      "classmap": ["./"]
     }
-}
+}