Updated janssonxx to work with v2.4 of jansson C library
- Fixed load_file() and load_string() to optionally include new 'flags' parameter

Other fixes:
- operator= allows appending new items to arrays
- operator<< appends null terminator and avoids compiler errors on VS 2008
- Unit tests to ignore iterator ordering (not guaranteed by jansson according to API docs)
- Unit tests use consistent JSON_COMPACT output format for verification