Twitter refactoring

 - Refactor pubsub stuff.
    Add a class `Pubsub` parent of classes `Pusher` and `Pubnub`.
    Better credentials handling among other things

 - Add StreamManager to module `tweets`.
    - Streamline class `Tweets`.
    - Add module `dictutils`.
    - Add generator wrapper `burst_generator` to module `decorators`.

 - Add `get_or_create` function to `ccouchdb`.
    
 - Refactor decorators to have them work on methods.
    Some decorators are now applicable to both functions and methods.