Parse PT_NOTE segment and expose it with the new '-n' option to readelf.

The PT_NOTE segment includes a list of notes with a header, name and
description. GNU ld includes in this segment the GNU build-id and GNU
ABI version information.

The '-n' or '--notes' option to readelf displays the notes contained
in the ELF file.
Merge pull request #48 from deymo/elf_note

Parse PT_NOTE segment.