[Design] merging modules across files

[mhegazy]

file1.ts:

module X.Y {
    export class Point {
        constructor(x: number, y: number) {
            this.x = x;
            this.y = y;
        }
        x: number;
        y: number;
    }
}

file2.ts:

module X.Y {
    export module Point {
        export var Origin = new Point(0, 0);
    }
}

Expected (old compiler):

Module 'Point' cannot merge with previous declaration of 'Point' in a different file 'class.ts'.

Actual (new compiler): no error

[ahejlsberg]

Fixed by #33.