Register object classes on plugin activation and unregister them on deactivation

Classes left behind in the database after plugin deactivation were giving a notice
(e.g. "Class 'Event' was not found").
Adds .gitignore that ignores all files beginning with a dot
Refactoring for the event editing form

- Splits the form into multiple smaller view
- Gets rid of the HTML tables
- Separates PHP and HTML code a bit
Merge pull request #56 from juho-jaakkola/refactor_edit_form

Refactoring for the event editing form
Flattens the structure of edit action and removes trailing whitespaces
Simplifies icon handling code