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.

I've added -Wextra as well as removed some of the -Wno-* flags and fixed issues in the code.

Also fixed the cmake build to put user-defined flags after default flags so that they can be overridden.