modify: 1. add permission tree 2. can be used as menu

[carsonlius]

permissions table add two field ,one is parent_id then wo can generate permissions tree;
another field is is_show then we can control menu

[coveralls]

Coverage remained the same at 63.636% when pulling d637b9b on carsonlius:5.5 into 825786f on ultraware:5.5.

[Ricky-rick]

I do not think this is very useful for this package. You can create your own tables to hold this information.

Schema::create('permissions_detail', function (Blueprint $table) {
    $table->increments('id')->unsigned();
    $table->increments('permission_id')->unsigned();
    $table->enum('is_show', [1, 0])->default(0)->comment('determine this permission is show when it is used as menu');
    $table->integer('parent_id')->default(0)->change();
});