Added platform selection to API login

I don't mind if you use this or not but I needed to be able to specify the platform at login. I have one account with two platforms (Personas) and the original code was always choosing the 360 one.

Firstly, for your original `LoginAsync` routine it will now look at your `<preferredPersona>` information returned at login time. This will be the last account that you logged in with. Your current code always chooses my 360 persona first and never my PC persona.

Alternatively there is a new overloaded `LoginAsync` method which takes platform as an argument. *This should really be an enum, rather than a string argument but I had bigger problems to solve (secure cookies problem)*

Lastly, I exposed and returned the `Persona` so that after login you have access to at least your persona id tag for displaying somewhere.

Anyway, I hope you find this useful. Thanks for a great API. Also its worth noting that your API works fine with mono 3.0.9, too.

Peace.