Destructuring

This pull request adds support for ECMAScript 6 destructuring declarations and assignments with down-level ES3 and ES5 code generation. For example:
```typescript
var [x, y] = [10, 20];
[x, y] = [y, x];
```
The above example generates:
```javascript
var _a = [10, 20], x = _a[0], y = _a[1];
_b = [y, x], x = _b[0], y = _b[1];
```
Another example:
```typescript
interface NameAndLocation {
    name: string;
    location: [number, number];
}

function getNameAndLocation(): NameAndLocation {
    return { name: "top-left", location: [10, 20] };
}

function displayNameAndLocation({name, location: [x, y] = [0, 0]}: NameAndLocation) {
    console.log("name=" + name + " x=" + x + " y=" + y);
}

var {name, location: [x, y]} = getNameAndLocation();
displayNameAndLocation(getNameAndLocation());
```
The above example generates:
```javascript
function getNameAndLocation() {
    return { name: "top-left", location: [10, 20] };
}
function displayNameAndLocation(_a) {
    var name = _a.name, _b = _a.location, _c = _b === void 0 ? [0, 0] : _b, x = _c[0], y = _c[1];
    console.log("name=" + name + " x=" + x + " y=" + y);
}
var _a = getNameAndLocation(), name = _a.name, _b = _a.location, x = _b[0], y = _b[1];
displayNameAndLocation(getNameAndLocation());
```
Work still remaining:
* Support for rest elements in array destructuring.
* Pure ES6 code generation with no rewriting (under -target ES6 switch).
* Updated spec.

Not crazy about those. Regarding getTypeFromBindingElementParent, we're not actually getting the type from the element's parent, we're getting it from the corresponding property or element in the type of the value being destructured. Regarding getTypeFromBindingElementInitializer, the type comes from the initializer or, lacking one, if the element is itself a binding pattern, from the shape of that pattern and recursively from it's elements.