Implement automatic hexadecimal decoding for xml_attribute::as_int and xml_text::as_int. This is effectively a form of strtol with base 0, but without octal support.

git-svn-id: http://pugixml.googlecode.com/svn/trunk@958 99668b35-9821-0410-8761-19e4c4f06640
Replace offsetof with sizeof since some compilers don't recognize offsetof as a compile-time constant expression

git-svn-id: http://pugixml.googlecode.com/svn/trunk@959 99668b35-9821-0410-8761-19e4c4f06640