Add custom logo using the variable theme_image already present in old themes

I added the feature to use a custom logo in the front page, this feature was present in mostly the same way (using the same vars) in the old play theme.
To accomodate the logo I widened the login box 100px altough it's not strictly necessary.
If you wish I can adapt the other themes too.
NOTE: It's the same pull request as #196, but with the commits properly squashed