Use the standard way of accessing the grandparent

The non-standard (easier to read) `$this->getInnerIterator()->current` does not seem to work in php 5.3
Don't reuse the DatePeriod object

php 5.3 does not automatically rewind the DatePeriod object, which can cause hard to debug errors.
Merge pull request #112 from mfairchild365/DatePeriod

Better support for 5.3