Optimizes Usergroup-display

This commit adds three things:

1. Support for 2-letter Countrycodes as defined in ISO 3166-1 alpha2. For that I added a list of country codes from
http://en.wikipedia.org/wiki/ISO_3166-1_alpha-2#Officially_assigned_code_elements
The list is added in the file ```include/countries-alpha2.inc``` which has been included in the ```include/site.inc```-file
2. Caching of retrieved usergroup-information. For that the list of
usergroups as well as the iCalendar-files of the single usergroups are
loaded once every 24 hours into files on the filesystem. Those files are
then used for information retrieval. The Webserver has to have write-permissions to the ```backend```-folder!
3. Slight optimization of UG-display.
For that I've changed display-style from an unordered to a definition
list. I've also set the date before the meeting-information as that
visually calms the whole list a bit.