Add Folder.findByInventoryPath

Hi Rich/Christian,

  this trivial patch adds support for `findByInventoryPath`. Useful when, for example, you need to find all the entities located in a specific inventory folder. It's based on 9e6d3eb79fbe1c5f3911e3715fcc038275062fe6 which should correspond to what you get when you `gem install rbvmomi`.

  Thanks for your work. It allowed us to migrate a couple webapps from Windows, Ruby and a mess of PowerShell scripts, to just Linux and Ruby. Along with `RVC`, `rbvmomi` really makes my ops life easier. :)

cheers,
 --
Giuliano