Email Settings refactoring and bug fixes

- Various bug fixes for email settings and event states (#286 and #287)
- Email Subjects now mandatory if send mail is enabled
- E Mails will be send if just they just have subject (body not mandatory!)
- Added template variables for venue and dates (registration, conference)
- Load Default template feature implemented (#16)

![Default template](http://s1.directupload.net/images/140720/gv7u9tie.png)

I will close the old PR ...