add home page
1. move admin link to application.html
2. add photo of product.
add photo model for store multi image in one product
add admin/product show
using simple_form
add new link in index page of products
add nav bar
custom devise sign in layout with bootstrap and simple_form
admin/product: fix show page error, if product without photo
add sign-in style which from example of bootstrap into application.css
adm/product: modify layout of show page
adm/product: modify layout of new page
adm/product: modify layout of index page and update TODO
admin/products: add edit, destroy and update actions
admin/product: add price
product add validate attributes
only admin can get into management page