Update comparison blog post to note that proto3 removes unknown field retention, for some ridiculous reason.

Unfortunately, commit 10487060 missed a case of CPU amplification via struct lists with zero-sized elements.

See advisory: https://github.com/sandstorm-io/capnproto/blob/master/security-advisories/2015-03-05-0-c++-addl-cpu-amplification.md

Details: https://github.com/sandstorm-io/capnproto/tree/master/security-advisories/2014-03-02-0-all-cpu-amplification.md

Details: https://github.com/sandstorm-io/capnproto/tree/master/security-advisories/2014-03-02-1-c++-integer-underflow.md

Details: https://github.com/sandstorm-io/capnproto/tree/master/security-advisories/2014-03-02-0-c++-integer-overflow.md

- Remove obsolete gittip buttons.
- Remove G+ follow crap.
- Indicate that Cap'n Proto is now a sub-project of Sandstorm.io.
- Add Twitter link.
- Update roadmap to not be obsolete.

Failed to notice this because only the cmake build refers to it, and this only makes cmake fail on installation, which was never quite tested.