Added structured error reporting to Reader.

This allows applications for interactively viewing or editing JSON to do
a better job of highlighting errors. Also added offset accessors to
Value, offering the same sort of functionality even for non-errors.

Thanks to Zach Clifford (zacharyc@google.com) for the patch.

git-svn-id: http://jsoncpp.svn.sourceforge.net/svnroot/jsoncpp/trunk/jsoncpp/src/lib_json@281 1f120ed1-78a5-a849-adca-83f0a9e25bb6

• json_reader.cpp[diff]
• json_value.cpp[diff]