Add phpfpm NixOS service module

This is an update and simplification of https://github.com/NixOS/nixos/pull/16 that works with the fpm support that @shlevy activated in `php54` (https://github.com/NixOS/nixpkgs/commit/6d5176a28bd953c74682ed13eaf0d23d013b4df6). It lets you specify the pool configuration as verbatim text, instead of building the config from Nix attributes. I think it is nice enough, and it brings maximum flexibility.

The following example works fine:

```
{ config, pkgs, ... }:

let

  nginxRoot = pkgs.runCommand "nginxroot" {} ''
    mkdir -p $out
    cat - > $out/index.php << EOF
    <html>
      <head>
        <title>PHP Test</title>
      </head>
      <body>
        <?php echo '<p>Hello World</p>'; ?>
      </body>
    </html>
    EOF
  '';

in {

  services.nginx = {

    enable = true;

    config = ''
      events {
        use epoll;
      }
      http {
        server {
          listen 80 default;
          server_name localhost;
          root ${nginxRoot};
          index index.php;

          location ~ \.php$ {
            try_files      $uri =404;
            fastcgi_pass   unix:/run/phpfpm/test;
            fastcgi_index  index.php;

            fastcgi_param  QUERY_STRING       $query_string;
            fastcgi_param  REQUEST_METHOD     $request_method;
            fastcgi_param  CONTENT_TYPE       $content_type;
            fastcgi_param  CONTENT_LENGTH     $content_length;
            fastcgi_param  SCRIPT_NAME        $fastcgi_script_name;
            fastcgi_param  REQUEST_URI        $request_uri;
            fastcgi_param  DOCUMENT_URI       $document_uri;
            fastcgi_param  DOCUMENT_ROOT      $document_root;
            fastcgi_param  SERVER_PROTOCOL    $server_protocol;
            fastcgi_param  SCRIPT_FILENAME    $document_root$fastcgi_script_name;
            fastcgi_param  PATH_INFO          $fastcgi_script_name;
            fastcgi_param  GATEWAY_INTERFACE  CGI/1.1;
            fastcgi_param  SERVER_SOFTWARE    nginx/$nginx_version;
            fastcgi_param  REMOTE_ADDR        $remote_addr;
            fastcgi_param  REMOTE_PORT        $remote_port;
            fastcgi_param  SERVER_ADDR        $server_addr;
            fastcgi_param  SERVER_PORT        $server_port;
            fastcgi_param  SERVER_NAME        $server_name;
            fastcgi_param  REDIRECT_STATUS    200;
          }

        }
      }
    '';

  };

  users.extraUsers.phpfpm = {
    description = "PHP FastCGI user";
    uid = 2222;
    group = "phpfpm";
  };

  users.extraGroups.phpfpm.gid = 2222;

  services.phpfpm.poolConfigs.test = ''
    listen = /run/phpfpm/test
    user = phpfpm
    pm = dynamic
    pm.max_children = 75
    pm.start_servers = 10
    pm.min_spare_servers = 5
    pm.max_spare_servers = 20
    pm.max_requests = 500
  '';

}
```