socket.io externs: promote export = const name:T to static class T

[haxiomic]

socket.io has this pattern

declare const SocketIO: SocketIOStatic;
export = SocketIO;

interface SocketIOStatic {
}

The interface SocketIOStatic shouldn't be generated (but is) and the export = SocketIO; should promote the variable SocketIO into a class of static fields

[haxiomic]

This seems to work correctly in 0.13.3, not sure on what's changed. SocketIOStatic is still generated but that's not a major issue