Ternary operator

This is a broadly useful computed property macro that I use in several apps, so perhaps it will be similarly useful to others

The idea is to do just about everything that a ternary operator can do. So, instead of 
```js
readyToDriveString: function () {
   return this.get('numTires') === 4 ? 'READY' : 'NOT READY';
}.property('numTires');
```
The developer could use this
```js
readyToDriveString: EmberCPM.Macros.ifThenElse('numTires', '===', 4, 'READY', 'NOT READY'),
```