Added AbsoluteUrlMixin and used it in any model.

cleaned up get_absolute_url() in some models
Merge pull request #1172 from ostcar/get_absolut_url_mixin

Added AbsoluteUrlMixin and used it in any model.
Added a poll description field for each assignment poll

fixed signals in MainMenuTests
Merge pull request #1170 from ostcar/poll_description

Added a poll description field for each assignment poll
Added possibility to use custom templates and static files in user data path directory.
Merge pull request #1169 from normanjaeckel/Loader

Added possibility to use custom templates and static files in user data ...
Used entry points to detect plugins automaticly.
Merge pull request #1156 from normanjaeckel/AutoPlugin

Used entry points to detect plugins automaticly.
Merge branch 'stable/1.5.x'
Rename and clean up static direcories.

- rename static subdirs to css and js
- move global static dir to core/static, changed
- move global templates dir to core/templates
- check comments and code style of all our own CSS and JS files
- minor changes related to the changes of template and static files