Move instaweb to a subpackage
    
    Gives possibility to add dependencies for git-instaweb http daemon,
    without having to install all dependencies at each git install.
    
    Currently, lighttpd is required by the git-instaweb package.
    The git-instaweb script supports other httpd daemons (httpd, mongoose,
    plackup [in perl(Plack)], and webrick [in rub-libs]).  lighttpd is the
    default, works without any configuration, and is only ~1M installed.