Include null attributes in XML

Note that this branch incorporates the changes in #66 which switched the tests over to PHPUnit. 

To change a plan's total_billing_cycles you'd need to set it to a null value:
> Number of billing cycles before the plan stops renewing, defaults to null for continuous auto renewal
http://docs.recurly.com/api/plans

But we don't send values that were set to null so we'll need to make a couple of changes.