Warn about created_at and updated_at

[ftrotter]

When DURC is compiling, it should create a list of warnings, which should be the last thing displayed by durc.php (it will take some doing to surpress later outputs).

It should catch things like "There is no CURRENT_TIMESTAMP on db.something_something.created_at"

Or
"There is no created_at field at all for db.something_else"

These warnings should show in the artisan steps.