add hooks to allow the oauth flow to be split up across multiple calls s...

Hey Simon,
I think it'd be useful (I know it is, at least, for a project I'm working on) to be able to 'split' the oauth flow up a bit so that it's easy to add hooks for apps trying to integrate your library to use other ways to get the oauth code.  Basically I just split step1 and step2 across two different methods and made the current perform_oauth leverage those.

One semantic difference in my diff is that the 'flow' variable is now a static member of Musicmanager, whereas before you were able to keep it as just a temp variable in the static perform_oauth method.  I wasn't exactly sure of the desired behavior there, so I just took a stab at it this way and figured you could give me some feedback.