Add clone of SBJson v4 spec so v3.x and v4 can be both be installed

This is identical to SBJson v4 except it can be installed alongside SBJson
v3.x. This is handy if you want to use version 4 but rely on a library that
depends on a previous version.
Merge pull request #6822 from stig/sbjson-4

Add clone of SBJson v4 spec so v3.x and v4 can be both be installed