Allow easy configuration of background/logo

We'd like to add some branding to our install of graylog by replacing the background image on the login screen (as cool as it is), and the graylog logo in the search bar. According to google, the only way to do this is to compile our version of graylog from github, something we'd rather not do.

It would be nice if those images could be specified in a configuration file.

Thanks for putting together a great product!