Proposal for enabling shorthand notation for object literal properties

This allows ES6 shorthand object literal property definition.  This addresses #418. e.g.:

```TypeScript
var  a;
var x = { a }; // equivalent to {a : a }
```
Notes:
* Only identifiers are allowed, keywords are disallows ``` { var } ```
* Qualified names are not allowed ``` { a.b } ```
* Any rewriting that would normally happen on the initializer should still occur. e.g. 
```TypeScript
module m {
    export var x;
}
module m {
   var y = { x }; // should emit { x : m.x }
}
```