Added possibility not to forward _make_packet errors

[vgavro]

if there is some error in FluentSender._make_packet - it's forwarded to fluent host instead of adding possibility to get local exception before forward. Added option for this forward_packet_error - which defaults to True for backward-compatibility.

Also EventTime creation (for nanoseconds precision) moved to _make_packet - just to more easily implement some custom logic for bulk sending..

[coveralls]

Coverage decreased (-0.3%) to 99.726% when pulling 0468c7d on vgavro:no_forward_make_packet_error into ace80f4 on fluent:master.