Twiddle the set of warnings that we test against in super-test.sh.

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.
Merge branch 'master' of github.com:kentonv/capnproto
Fix test script ugh.
Tweak installation documentation.
Merge pull request #160 from pqu/doc

Tweak installation documentation
Document new features in 0.5: Generics and canonicalization.
Merge branch 'master' of github.com:kentonv/capnproto
Add missing file to Makefile.am.

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.
Last-minute doc fixes.
Release script fix.