Update of the Tweet models, skeleton of searcher

I updated the tweet models based on the various constraints I encountered while
programming. I also added a skeleton for the search API, which will be
completed soon.
Tweet Searcher works

- Implementation of a tweet searcher with call back. The searcher
ping the Twitter API at a period frequence to get new tweets.
- Skeleton for the TweetManager class, which will launch both :
	- The Tweet Streamer (Streaming API)
	- The Tweet Searcher(Search API)
Modification of listeners to be Actors

Since the computation based on the tweet might take some times,
I changed the way the tweets are received. They are now
send to another actor, called listener. This one will then be able to
handle the tweet as wanted.