Merge pull request #412 from NicoleRauch/master

Activity dates and times will now be validated.
included data tables to make administration tables sortable
only execute extension if data tables are present
Merge pull request #413 from leider/master

	included data tables to make administration tables sortable
removed useless code, now that start and end are checked by the client
Merge pull request #414 from leider/master

removed useless code, now that start and end are checked by the client
Update README.md
Hotfix for #415
fix typo on goodby
Merge pull request #418 from rtack/master

fix typo on goodby
Merge commit '32937ac960807953879717f29cc815998cf3c6de'