Patch for v2.4 of jansson C library

Hi there!

I just forked your repo to update it to work with v2.4 of jansson C library.  It should still be backwards compatible with prior versions of jansson.

This includes:
- Fixed load_file() and load_string() to optionally include new 'flags' parameter

Some other fixes I included:
- 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