Transform [login] property into source event (event handler)

[ADjenkov]

Consider changing the [login] property to one-way to source event handler. I personally see two inconveniences with the current implementation:

1. It is misleading, for me, to bind function which will receive a special set of arguments (Facebook.LoginResponse) to a property with [eventHandler] instead of (eventHandler)

2. I think there is a problem with the context inside the callback function (the one that is sent to login-manager). The this is not the desired one.