Use Slim 3

Since Slim 3 doesn’t have built in view rendering, added a class for
that. Also moved various services and functions into the DI container.
Services that used $app before now properly rely on only the resources
they need (e.g. the request object).

There are still a few rough edges on Slim 3 (e.g. lack of native cookie
support on the response…hence the copy-pasted function from
Slim\Http\Cookie). But it’s stable enough that this works just fine :)

composer.json

@@ -10,8 +10,8 @@
     }
   ],
   "require": {
-    "slim/slim": "2.*",
     "twilio/sdk": "dev-master",
-    "aura/sql":"2.*"
+    "aura/sql":"2.*",
+    "slim/slim": "3.x-dev"
   }
 }