Encode length using pack 'V' instead of 'C4'

Instead of doing the bit-shifting by ourselves, use pack's 'V' encoding to
encode the length as unsigned long (32-bit) in "VAX" (little-endian) order.
Update documentation of zabbix_template_1_8
Test _encode_request

With a focus on the correct length calculations.
Remove broken link

Didn't find any replacement either.
bin/zabbix-sender: Fix default port, add --hostname
Merge pull request #1 from micha137/master

Length encoding using pack 'V'