Avatar optimization

This was inspired by: http://custom.simplemachines.org/mods/index.php?mod=1924  I sent Aldo a pm asking for his permission a few weeks ago but haven't got any response yet, it's basically his code with a few changes but there is not that many ways to do this anyway.

Doesn't handle utf8 nor handles browser specific headers. The change in index.php is ugly but since the whole point of this is saving resources creating a new action is pointless as is the same as leaving the avatars been handle by Display.php

Don't forget to set a proper upload dir on install/upgrade since right now there isn't one by default so there is an error when you try to upload an avatar. The error triggers an undefined error string so I added a dummy one:  $txt['attachments_no_write'] = 'The attachments directory is not writable';

I also added a few more useful extensions to .gitignore, mostly for those of us who uses windows.