Fixes behavior for disabled zone-statistics
Fixes behavior for disabled zone-statistics

When complete "zone-statistics" aren't enabled in the BIND configuration
("full" or "yes" depending on the BIND version) no counters are present in the
XML structure provided by BIND. Up to now pybindxml failed parsing such a
structure, as it expected counters being available for all zones.
This commit fixes that by making these counters optional.

Dunedan/pybindxml,Dunedan/pybindxml,jforman/pybindxml,jforman/pybindxml