add new public functions to jansson.def
Merge pull request #141 from chipdude/nuls

add new public functions to jansson.def
Document s% and s# for json_pack, s% for json_unpack
Add Chip's changes to release notes
load.c: Change an error message
Fix NUL byte check for object keys
Clarify json_string_length doc
Add JSON_ALLOW_NUL decoding flag for enabling NUL byte support
Document JSON_ALLOW_NUL and clarify NUL byte handling