Minor fixes to comments in rpc.capnp

Remove a reference to Main, add a comment about the legality of acceptFromThirdParty, and fix a few typos and one pervasive grammatical error.
Remove obsolete MSVC stuff.
Fix upload step of release script.
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.
Fix test script ugh.
Tweak installation documentation.
Document new features in 0.5: Generics and canonicalization.
Set release branch version to 0.5.0-rc2.