Add "Protect your privacy" page

This pull request can be previewed live here:
http://bitcoinprivacy.zapto.org/en/protect-your-privacy
http://bitcoinprivacy.zapto.org/en/you-need-to-know

This page gives more explanation about the limits of Bitcoin privacy, many concrete ways to increase privacy and also mention potential future improvements related to privacy (payment API, zerocoin, manual input address selection. etc.)