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: https://svn.code.sf.net/p/jsoncpp/code/trunk/jsoncpp@281 1f120ed1-78a5-a849-adca-83f0a9e25bb6