Corrected the image_resize script. It used to make an infinite loop somehow on GIF's which have no transparent color.
Removed the creation of the folders from the setup because i couldnt know exactly the location of the placeholder image so that the script wouldn't need to be run at all. It would be easier if the folders would be sent with the commit because they would not affect the real site(the folders already exist). So a fresh install would not need to run this script. Did some minor changing to the code(some lines were longer than 80 characters).

An user cannot change his avatar through the attachment page(an error will tell him to change it from his profile page) but he can delete it from there and from his account page too(there is a shortcut). 

An avatar can only be jpeg, png or gif so i added a different array with these mime_types. The others are yet to be permitted.