Merge pull request #2 from tw-preschool/sancoder

SanCoder: add basic login feature
Merge pull request #3 from tw-preschool/sancoder

SanCoder: add db and server task bash file
Merge branch 'sancoder'
Merge pull request #4 from tw-preschool/sancoder

SanCoder: add style for login.erb and admin.erb
Merge pull request #5 from tw-preschool/sancoder

SanCoder: add a message prompt when an error occurs during the login
Merge pull request #6 from tw-preschool/sancoder

SanCoder: fix the rakefile bug
zj:view all existing products
yjy:add a edit page in app.rb
yjy:add "edit" and "delete" button in admin
yjy:add edit.erb
add promotion into table products and clear some useless code
adjust gap
change span#count 2 to 0
modify items.html
SanCoder: fix a bug using back slash that should use slash in login.erb