Refactor decorators to have them work on methods.

Some decorators are now applicable to both functions and methods.
Add get_or_create function to ccouchdb.
Add StreamManager to module tweets.py.

- Streamline class `Tweets` of *tweets.py*.
- Add module `dictutils`.
- Add generator wrapper `burst_generator` to module `decorators`.
Refactor pubsub stuff.

Add a class `Pubsub` parent of classes `Pusher` and `Pubnub`.

Better credentials handling among other things
Merge pull request #17 from ooda/twitter-refactoring

Twitter refactoring