Accumulated fixes and enhancements

These are the accumulated changes that I made to the plugin to get it working for a client.
Bug-fixes
- use correct variable for user frequency in canSend()
- use id instead of date/time added to select rss items (because several items can have the same added value)

Enhancements
- tidy-up use of $rssfrequencies array
- add hourly frequency
- display items in reverse id order - most recent first
- cosmetic layout changes