add post not found error handling, extra links for next lessons in index
add lesson 4 index
add cookies to lesson 4 to track visits
not really sure
add sinatra-contrib to gemfile
add Gemfile.lock to gitignore
Revert "add cookies to lesson 4 to track visits"

This reverts commit cb62fe0610051968f316fd94fbe2ad321bfdd605.

Conflicts:

	views/lesson4/index.erb
	web.rb
fix bug with datamapper default database