Adapted Session to the new Credentials class

darcs-hash:20090518140559-58103-f786896faad71ea5f9ad773234772f39690921c2
Refactored Credentials object and added from_uri argument to some classes

- Credentials object now only has "username" and "password" attributes
- Invitation, Request and Subscription now take "from_uri" as their first
  init argument
- Passing credentials when creating these objects is optional

darcs-hash:20090518140729-58103-e1f6437a480f73daae0428910f94be8259176358