Improve install command, add custom migrations system

Implemented our own migration repository + migrator (based on Laravel's
stuff) so that we can keep track of which migrations have been run for
core and per-extension. That way we can simple call the migrator to
upgrade core/extensions, and to uninstall extensions.

migrations/2015_02_24_000000_create_notifications_table.php

@@ -1,8 +1,7 @@
 <?php
 
-use Flarum\Install\Migration;
+use Flarum\Migrations\Migration;
 use Illuminate\Database\Schema\Blueprint;
-use Illuminate\Support\Facades\Schema;
 
 class CreateNotificationsTable extends Migration
 {
@@ -15,7 +14,6 @@ class CreateNotificationsTable extends Migration
     public function up()
     {
         $this->schema->create('notifications', function (Blueprint $table) {
-
             $table->increments('id');
             $table->integer('user_id')->unsigned();
             $table->integer('sender_id')->unsigned()->nullable();