switch from linear congruential to Xorshift generator

Error messages contain line numbers and new license comment is two lines shorter...

For portions currently copyright by Kenton (most of it), transfer copyright to Sandstorm Development Group, Inc. (Kenton's company).

The license change is practically meaningless, as MIT and BSD 2-clause are legally equivalent. However, the BSD 2-clause license is sometimes confused for its ugly siblings, BSD 3-clause and BSD 4-clause. The MIT license is more immediately recognizeable for what it is.

Rémy Blank and Jason Choy (the two non-trivial contributors) are on record as approving this change:

https://groups.google.com/d/msg/capnproto/xXDd2HUOCcc/gbe_COIuXKYJ

Use /usr/bin/env shebang - /bin/bash is not universal

On FreeBSD bash is /usr/local/bin/bash.

Update doc/README.md to host documentation server

Fixed two examples

Close #94: obsolete comment in rpc.capnp

fix help message in capnpc-c++

fix typos and remove unused declaration