Adding support for tuple types (e.g. [number, string])

This commit adds support for typle types. A tuple type is written as a comma separated sequence of types enclosed in square brackets:
```typescript
[T0, T1, ... , Tn]
```
A tuple type corresponds to an object type that extends `Array<T>`, where `T` is the best common type of the tuple element types, with a sequence of numerically named members:
```typescript
{
    0: T0;
    1: T1;
    ...
    n: Tn;
}
```
When an array literal is contextually typed by a tuple type, each of the array literal expressions are contextually typed by the corresponding tuple element type, and the result is a tuple type:
```typescript
var t: [number, string] = [1, "hello"];
t = [];                 // Error
t = [1];                // Error
t = [2, "test"];        // Ok
t = ["test", 2];        // Error
t = [2, "test", true];  // Ok
```
When a value of a tuple type is indexed with a numeric constant, the resulting type is that of the corresponding type element. For example:
```typescript
var x: [number, string] = [1, "hello"];
var x0 = x[0];  // Type number
var x1 = x[1];  // Type string
var x2 = x[2];  // Type {}
```
A tuple type is assignable to a compatible array type. For example:
```typescript
var a1: number[];
var a2: {}[];
var t1: [number, string];
var t2: [number, number];
a1 = t1;  // Error
a1 = t2;  // Ok
a2 = t1;  // Ok
a2 = t2;  // Ok
```
Type inference works as expected for tuple types:
```typescript
function tuple2<T0, T1>(item0: T0, item1: T1): [T0, T1] {
    return [item0, item1];
}
var t = tuple2("test", true);
var t0 = t[0];  // string
var t1 = t[1];  // boolean
```
Next step is to support EcmaScript 6 style destructuring of objects, arrays, and tuples.
not sure what this is doing in the context of tuples.  Can you clarify why this is necessary.