Cron: need a last cron run variable and a cron test (usability)

As I was working on a module I realized that Drupal doesn't keep track of the last cron run, which is strange, since it adds a watchdog() after every run but nothing keeps track of the time of the last run. So this patch fixes that and as a result, cleans up a few core modules to make use of this new variable.

Additionally, since we can keep track of the last cron run, it makes sense to test to see if cron is configured properly. Neil suggested this as a result of my proposed patch so I expanded it's capabilities to do just that. On the settings page there is new 'cron jobs' section which notifies the user of the status of cron: it's running, it hasn't run within the past 1.5 hours (Neil suggested this time check, I agree, Drupal installs probably benefit most from 1.5 hour checks), or it hasn't run at all. This really makes setting up cron a bit easier and also makes it easier to pinpoint any errors if cron for some reason or other stops running.