Major API change to use contect from code.google.com/p/go.net/context

Hi,

I'm not sure if you're willing to do such a large API change at this time. I find using context.Context cleaner (since the context is immutable) and there is builtin support for timeout and cancellation.

If you want to pursue this, I also have a middleware for sessions ready (requires a small modification to github.com/zenazn/goji/web). If you don't want to go this way with goji, no problem (I'll simply rename my fork to avoid any confusion since the API will be different).