Change grid.jpg

The old image looks not too good with the style of the site.
Image is from and subtlepatterns.com licensed under the CC BY-SA 3.0.
Update browse.php

Making browse.php actually work!
I'm not so into your code, so I hope everything is correct.
Update login.php and register.php

Username, password or email shouldn't be empty!
Merge pull request #21 from ftbastler/master

Change grid.jpg; update browse.php; update login.php and register.php