Retweet

Added
- use sqlite database through sqlalchemy to store already sent tweet ids.

Added
- retweets parameter to retweet only if a tweet already has been retweeted the number of times indicated by this parameter.
- waitminsecs parameter for the minimal seconds to wait before processing the next tweets.
- waitmaxsecs parameter for the maximal seconds to wait before processing the next tweets.

Added
- bump Python version to 3.4.
- improve pep8 compliance.

Added
- initial version.