Drop onlick() handlers in documentation, in favor of simple links.

Implement most of JsonCodec::decodeRaw

JSON: encode NaNs and infinite floats as `null`

Minor cleanup

setup-autotools.sh is no longer needed as of 6cd25260.

The instructions work without having Subversion installed as well.

This reduces dependencies on stdc++. For rationale, see
  https://github.com/kamalmarhubi/capnproto/commit/62a562f6d2c7bd147958106a1cf4169533d43f83#commitcomment-14375773

Requested by jo_liss on Twitter:

https://twitter.com/jo_liss/status/664884038252027905

(Also implement defaultValue<>() helper which was defined but apparently never implemented.)

Such values are not allowed by the JSON spec. We match the behavior of
JSON.stringify in Firefox and Chrome.

fixes https://github.com/sandstorm-io/capnproto/issues/261

Per spec.

This includes:
- ensuring we don't go off the end of input if it's not null-terminated
- checking for overflow and underflow
- being more careful to check that numbers match JSON lexical syntax