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.

Having very complex conditionals like this forces all readers to internalize all the possible complex AST shapes, and try to determine if they're validly represented here.  Consider extract out helper functions with clear comments that explain the different variations that can happen here, and why these checks are correct and exhaustive.