adds support for user callback to pull session key from request

hey guys,

i recently found myself in the position of wanting to put a request's session key in the request parameters rather than in a cookie. This was in the context of writing a test client for a RESTful application, where i'm switching sessions quite a lot- it felt somewhat cleaner to treat the session key as a parameter, rather than something special that requires its own header.

i added a few lines of code to the session middleware which lets the constructor accept a 'get_key' option, which should be a callback to be called when connect wants its session key. if nothing is specified, the default behavior is retained (that is, just use the cookie).

thanks!