Saves created_at also as a timestamp in the `tweets` collection
Adds script to migrate current database
Changes tweet index to contain `created_at_timestamp` instead of `created_at`
Updates docstring for the `Filteredcapture` listener
Removes code that was commented out
Avoids cursor timeout in the tweet migration script

We have a very large collection, and the default cursor timeout is 10 minutes.
According to the docs[1], `timeout=False` will make sure the cursor never times
out:

> timeout (optional): if True (the default), any returned cursor is closed by
> the server after 10 minutes of inactivity. If set to False, the returned
> cursor will never time out on the server. Care should be taken to ensure that
> cursors with timeout turned off are properly closed.

Thank you @fccoelho for pointing this out.

[1] http://api.mongodb.org/python/current/api/pymongo/collection.html#pymongo.collection.Collection.find
Uses a logger instead of just writing to stdout
Logs any exception that happens during parsing or inserting in `add_timestamp_to_existing_tweets.py`
Merge branch 'feature/add_created_at_timestamp_to_tweets'
Adds more logging to the the `TwitterStream.py` script