Refactor formatting from view_form intor renderer

Moved code to format a group from view_form.php into renderer.php.
Added a language string for a groups description.
Improve formatting for groups

Added a general box around the group informations and moved improved
the positioning of the hr-tags.
Add function to list all memberships for a user

This is usefull to show a student all of his automatically assigned
groups.
Show groups before and after rating period

Students can now see all rateable groups before the rating period
begins.
They can also see the groups into which they have been assigned after
the rating period has ended.
Remove some occurences of the word 'group'

We should not force the word 'group' onto the plugin users
and students, because they may call it 'tutorial' or 'seminar'.
Remove commented lines
Improve language strings for rating prompt

Added a short and long version of every rating.
Some parts in the code and stylesheet had to be changed,
because the language strings got renamed.
Stop showing groups name twice during rating
Special treatment for unix time less than one

Don't show Jan 1. 1970.
Merge pull request #2 from stefankoegel/master

Usability improvements