Replacing __defineGetter__ and __defineSetter__ with Object.defineProperty get and set

Methods __defineGetter__ and __defineSetter__ are deprecated (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineGetter).  This is also what's stopping Arbor from working on IE9 (confirmed in my branch).

I have replaced the deprecated methods with Object.defineProperty for:
Node.prototype.p (and all contained within)
Node.prototype.fixed
Node.prototype.tempMass
Node.prototype.mass

This has been tested with the Atlas demo on Firefox 27.0.1, Chrome 24.0.1312.56 and IE9.