Add static library and install targets
Fix json_parse, which didn't set J->root on success

json_parse() never sets J->root, so chunk-wise parsing can never be
completed. The symptom of this was json_print() and friends silently
failing after chunk-wise parsing, because root was NULL.  json_parse_()
works correctly, for all use cases, so use it.
Add some undocumented cmds to the usage message.
Ignore build artifacts
Add top-level Makefile with install target

Easy of packaging requires a top-level makefile with an install target.