re-organize imports/files
reader/writer: evented refactoring

* proper separation of responsibility, introduce Client
  superclass, move work into AsyncConn, etc.
* add EventedMixin to provide on(), off(), trigger()
* switch Reader/Writer
travis no longer supports python2.5
Merge pull request #52 from mreiferson/refactor_52

refactor
reader: add output buffering feature negotiation
Merge pull request #49 from mreiferson/ob_49

support output buffering feature negotiation