Merge pull request #1 from zxlai/develop

Develop
Guardfile
models/post.rb: add validate, change after_save to before_save
Update Gemfile
Guard
Header
Remove unte test
Change header name
post_controller.rb: 1. add a action to check whether current user owns the post. 2.add before_action: authenticate_user!
views/posts/_form.html.erb: 1. add option radio_button. 2. add hidden_field, current_user.id.
views/posts/index.html.erb: 1. add notice message. 2. change td post.user -> post.user.username.