Merge branch 'modeling-users'
Make home page
Merge branch 'static-pages'
Add function of sign-in
Add function of sign-out
sign in after sign up
Merge branch 'sign-in-out'
Modeling friends
Add function of make friend
Add function of un-make friend
Add spec and refactoring spec
Refactoring friend form
Merge branch 'friends'
Add unique constraint to Users