Add Get(), Post(), Put() and Delete() helper methods to the Session type

A quick fix to make the Session type consistent with the doc and with the package-level functions. At the moment, this code in Scenario 3 in the doc doesn't compile as the Session type is missing those helper methods:

    res, _ := session.Get("/me/feed", nil)

This pull requests fixes that. And those helper methods are quite handy is any case.