Managed Entity doc bug

Demonstrates that pyVmomi acts slightly differently from other bindings to
vSphere. This test demonstrates that optional properties are always present so
that the test `hasattr(thisObj, 'parent')` need not ever be used.

closes https://github.com/vmware/pyvmomi/issues/104
Merge pull request #106 from hartsock/issues/104

Demonstrates proper use of Managed Entity parent attribute