Implement create post page 

1. Add constraints to fields at create new post page
2. change the layout of create new post page
3. handle create post failure, show error message
4. change server to unicorn, use ssl in production
5. show create post successful message at new post page
6. change routes.rb, use posts for posts/index, use new_post for posts/new
7. need to write tests for create new post page