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.

include/json/value.h

@@ -442,6 +442,13 @@ namespace Json {
       iterator begin();
       iterator end();
 
+      // Accessors for the [start, limit) range of bytes within the JSON text from
+      // which this value was parsed, if any.
+      void setOffsetStart( size_t start );
+      void setOffsetLimit( size_t limit );
+      size_t getOffsetStart() const;
+      size_t getOffsetLimit() const;
+
    private:
       Value &resolveReference( const char *key, 
                                bool isStatic );
@@ -509,6 +516,11 @@ namespace Json {
       int memberNameIsStatic_ : 1;       // used by the ValueInternalMap container.
 # endif
       CommentInfo *comments_;
+
+      // [start, limit) byte offsets in the source JSON text from which this Value
+      // was extracted.
+      size_t start_;
+      size_t limit_;
    };