add is_admin column to users table

Author: Gareth Redfern

database/migrations/2014_10_12_000000_create_users_table.php

@@ -17,6 +17,7 @@ public function up()
             $table->id();
             $table->string('name');
             $table->string('email')->unique();
+            $table->boolean('is_admin')->default(false);
             $table->timestamp('email_verified_at')->nullable();
             $table->string('avatar')->nullable();
             $table->string('password');