add a check for minOccurs == 0 when determining member nillable property

Currently `Variable.nillable` is set to true only when the wsdl element has the `nillable` attribute set to `true`. 
However this is not the only way (nor the typical way) of making fields accept null values. The "normal" way of making fields accept null values is by setting the `minOccurs` attribute to `0`.
See http://www.ibm.com/developerworks/webservices/library/ws-tip-null/index.html
I would appreciate a inline comment explaining this - perhaps even with a reference to the article you link to.

We could go as far as renaming the `$nillable` variable to avoid confusion. Given the current change it no longer refers directly to the XML term nillable but rather whether the member supports `null` values or not.