Plugin refactoring based on @tikaszvince's work

Add feed class for RSS functionality
Add uninstall.php to clean up after the plugin is deleted
Add support for Wordpress Multisite
Improve code readability
Fix references to old settings page (wpp_admin is now wordpress-popular-posts)
Cache data per instance (views, comments, stats) to avoid returning the wrong info when multiple widgets/shortcodes are used at the same time
Remove hardcoded plugin localization domain
Remove additional hardcoded plugin localization domain
Remove commented, outdated parameters from the FAQ
Update translation paths and singular/plural handling
Remove log table and all its references
Enable data caching on demand
Add check for interval value, if none is provided fallback to 1
Remove debugging echo code
Add missing fields on __update_views()
Fix date field getting the wrong date parameter
Switch *_option to *_site_option for Multisite support
Add back ability to clear tables from Settings page
Update translation strings
Update spanish translations
Fix undefined variable: d in line 1104
Tools section reorganized. Translation strings updated.