continuing work on the events feature. editing an event is now possible. Date and time can be selected using a picker with best_in_place. Users can join events.

trello: https://trello.com/c/cOa9EpLf/677-veranstaltungen
working on events feature:

* email invitations: test with own email address
* using `letter_opener` in development to display mails in browser
* add-event buttons

trello: https://trello.com/c/cOa9EpLf/677-veranstaltungen
avatars: adding rounded corners
events: accepting email joins via GET; adding group invitation delivieries.

trello: https://trello.com/c/cOa9EpLf/677-veranstaltungen
events: adding ical export.

first attempts to export events to ical. also prividing webcal feeds.

trello: https://trello.com/c/cOa9EpLf/677-veranstaltungen
events: events#index

* html feed for public events
* ics (webcal) feeds via events#index
* publish events on local and global public websites
* index events with datatables
* auth feeds via auth token

trello: https://trello.com/c/cOa9EpLf/677-veranstaltungen
events: to attend an event does not count as full corporation membership.

* modify `Role#current_member`
* add `Role#full_member`
* modify `User.current_corporations`
* fixing specs

trello: https://trello.com/c/cOa9EpLf/677-veranstaltungen
Merge branch 'master' into sf/events
events: addings specs and fixing issues.

trello: https://trello.com/c/cOa9EpLf/677-veranstaltungen
Merge pull request #58 from fiedl/sf/events

Veranstaltungen-Feature