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_config_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 CreateConfigTable extends Migration
 {
@@ -15,7 +14,6 @@ class CreateConfigTable extends Migration
     public function up()
     {
         $this->schema->create('config', function (Blueprint $table) {
-        
             $table->string('key', 100)->primary();
             $table->binary('value')->nullable();
         });