Option to disable the Referer Header

[Due to the discussion on tor-talk mailing list]

The Referer Header isn't disabled by default "because of the non-trivial number of sites that rely on the Referer header to authenticate image requests and deep-link navigation on their sites". Anyway, we need to add such option to the TorButton setting list just to make configuration more comfortable.

Look up for network.http.sendRefererHeader in about:config.