Merge pull request #481 from BGSU-LITS/master

Using array_key_exists when loading plugin paths
Use item_image_gallery() to use a default icon for non-image files.
Add and use a specific class for "Use HTML."

Fix #480.
Go back to using files_for_item() by default.
Fix #482, catch exception when guessing IM path.
Making sure that the plugin script directory is an array