Including validation to List model
lists error handling

Fixing duplicated flash alerts on application.html,create
shared/_form_errors.html to handle lists errors and include the error
messages in the lists/new.html
Include error message on lists/edit.html
Requiring login to see and create lists
Done. Now each user has his own list.

Changes on lists controller so each user has his own list
Create and then delete my/lists - realized that I don't need it
Merge branch 'list_authentication'