Replaced defineGetter with Object.defineProperty

Object.defineProperty is a cleaner API for defining object properties, and has better cross-browser support (i.e. IE9, which I needed to support in a recent project using Sideshow). Refer to http://web.onassar.com/blog/2011/05/30/definegetter-definesetter-in-ie9/ for more info. Thanks!
Merge pull request #5 from tonybaroneee/master

Replaced defineGetter with Object.defineProperty