Please review: suport nested custom headers (i.e. x-death)

This patch adds support for nested tables in envelope headers. For details, see issue #45.

Could someone please review it, I'm not comfortable with the allocation and stuff.
I'll add some tests for the new behavior soon, but until then could somebody more experienced please have a quick glance over it? I basically moved the table parsing in its own function (parse_amqp_table) so we can call it whenever we encounter a table.

Thanks in advance!