Notes

Added notes field to models Site, Event (see issues #3, #17).

No fancy WYSIWYG editing, just textareas.
If notes can be NULL, then calling str() on any empty values get displayed as the string "None". If someone adds notes and then deletes them, the value stored in the database doesn't revert to NULL, but to an empty string, so we don't get the same behaviour. It would be better to give the notes field a default value of an empty string, that way we only have one value that represents "no data".