Improve additional security measures (ghpages)

[luclu]

Investigate recommendations from https://observatory.mozilla.org/analyze.html?host=ethereum.org

• [ ] Content Security Policy (Content Security Policy (CSP) header not implemented)
• [ ] X-Frame-Options (X-Frame-Options (XFO) header not implemented)
• [ ] HTTP Strict Transport Security (HTTP Strict Transport Security (HSTS) header not implemented)
• [ ] X-XSS-Protection (X-XSS-Protection header not implemented)
• [ ] X-Content-Type-Options (X-Content-Type-Options header not implemented)

[evertonfraga]

1) http://qszhuan.github.io/technology/2015/08/12/add_csp_to_github_blog