solve issue #3

Duplicated codes in StreamSink, StreamSource, StreamTask has been
encapsulated into StreamComponentFactory. Test passed. Commented codes
may be removed if no further modification is needed.
Merge pull request #4 from Frank-Wu/operator_refactor

StreamComponent refactor
StreamComponent additional refactor
emit update
Merge branch 'master' of https://github.com/stratosphere/stratosphere-streaming