adding overlay wrapper for interactive experiments

Hi @hartsock,
This is my old pull request, rebased and with the pep8 issues fixed.
I also moved the module into the `tools` package, which is much more fitting as you mentioned.
I squashed everything into one commit.


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.