OpenSystemsDevelopment / qpdf

For std::string and std::vector, replace operator[] with at.  This was
done using an automated process.  See README.hardening for details.

Add QUtil::hex_encode to encode binary data has a hexadecimal string,
and use it in place of sprintf where possible.

Add virtual methods to QPDFObject, wrappers to QPDFObjectHandle, and
implementations to all the QPDF_Object types.

This method allows parsing of the PDF objects in a content stream or
array of content streams.