Adds a new column to tweets in db : invalid

Will be used to set problematic as crawled and try to correct them later on. This avoids having to process them over and over.

I might have to think about a better way to perform database updates if I do it often.
Tweet invalidation works.

Count now never tries to process twice the same tweets.

Next step :
Changes streamer to be a thread too, and stop it with a signal handler
The stream can now be closed properly.

Next objective is to be able to create/display a leaderboard for a given tag.
First ugly leader board is working.

What I have to do now is to order it a bit better