Add ManagedObject wrappers

Folder.Children and SearchIndex.Find methods convert
types.ManagedObjectReference to govmomi package types via the
newReference method.  Adding support other types such as:
StoragePod, ClusterComputeResource, VmwareDistributedVirtualSwitch

Fixes github issue #119
Merge pull request #121 from dougm/types

Add ManagedObject wrappers