json: Hide the input array from parser methods

Fix some outdated comments and error messages.

Add missing case analysis in handleUnimplemented().

Support extracting the current exception count on MSVC2015.

The MSVC runtime was heavily refactored, so the internal function we call to
get the current exception count changed for newer MSVC versions (as did the
offset).

Account for the possibility of adopting an empty struct.

This encapsulates the input array, hiding it from the parser's methods.
Access is forced to go through `advance` and `consume*` methods on the
new `Input` object. The aim is to make it harder to have errors like
https://github.com/sandstorm-io/capnproto/pull/270

adjust the set of white space characters to the set described by RFC7159

Check num remaining bytes of escaped json character

Prefer display:block over inserting new <p> elements.

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.)