Add User model that uses has_secure_password
Add ability to sign in.
WIP
Associate user to post and show associated user on post display
Fix tests that broke after associating user to post
Merge remote-tracking branch 'remotes/ben/week-four'

Conflicts:
	app/models/post.rb
	config/routes.rb
	db/schema.rb
	spec/features/deleting_a_comment_spec.rb
	spec/features/viewing_comments_spec.rb