cdap twitter pack initial commit

README due in next PR
Internally, this Flowlet uses a queue for the tweets that were pulled from the Twitter stream or read from a file and are
ready to be emitted. Pulling from the Twitter stream or a file happens asynchronously to the {@link #collect()} method.
The default maximum queue size is set by {@link #DEFAULT_INTERNAL_QUEUE_SIZE}. Client code can
override this default by passing a different value into the appropriate constructor.
Emitting is done in batches, with the batch size set by {@link #DEFAULT_EMIT_BATCH_MAX_SIZE}. Client code can
override this default by passing a different value into the appropriate constructor.