Added RSS feed for activities

Activities now can be accessed in RSS feed form after authentication.
(Useful for systems without e-mail.)

For example:
Browser: http://tinyissue.server/index.php/project/(:num)/rss
Wget: 
```
wget --keep-session-cookies --save-cookies cookies.txt --post-data 'email=email@provider.com&password=secret' http://tinyissue.server/index.php/login
wget --server-response --spider --keep-session-cookies --load-cookies cookies.txt -p http://tinyissue.server/index.php/project/1/rss
```