[BUGFIX] Bugfixes and optimisations

- When no parameter is recieved, router now returns null instead of empty
  string.
- json method in Response class now stops response after execution.
- Updated composer.json with correct autoload.

composer.json

@@ -20,7 +20,7 @@
     },
     "autoload": {
         "psr-4": {
-            "Pecee\\": "src/"
+            "Pecee\\": "src/Pecee/"
         }
     }
 }