Fix bug causing --dryrun to silently crash at end

Previously, pcap_close() was still called on a null pcap handle when
--dryrun was passed as option, then ZMap would silently crash on the
monitor thread, returning a non-zero exit value, but with no error
message printed to stderr
Add tests for sharding, update relevant .gitignore

Requires the Python module 'sh'

sudo pip install sh