Create a base class which wraps Elf64_Shdr

Previously Section() was both the wrapper around Elf64_Shdr and the
utility class for building up instances. This made it needlessly
complicated to initialise (it requires access to the string table and
section data).

A new class SectionHeader now adds acces functions to Elf64_Shdr,
while Section retains its role in building up sections.

This fixes issue #28