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_users_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 CreateUsersTable extends Migration
 {