Implement hook_requirements

Given the steep requirements of imagecache, we should implement hook_requirements to try to cut down on the amount of confusion with using imagecache.

This patch checks the following requirements during runtime:
- An image library is installed
- If using GD, individually checks for gif, png, and jpeg support (no warning issued however)
- Clean URLs are enabled
- Public Files are enabled

If any of these requirements are not met, the "Problem with your Drupal installation" warning is displayed on /admin and the problems are listed out.

We could also make it so imagecache isn't even able to be enabled, but such a route would require a hook_form_alter on the modules page. This patch seems like a logical place to start requirement checking.