adding overlay wrapper for interactive experiments

This makes simple experiments in an interactive python shell much
easier. After authenticating to a vCenter host interactively, it
is possible to directly iterate over all VMs with very little code.

Wrappers are provided for convenience, though they passthrough any
undefined fields or methods to the underlying VIM object.
Merge pull request #93 from mriehl/sample/simple-interaction

adding overlay wrapper for interactive experiments