Change model to fit sql naming conventions. Add foreign keys or relationships between statuses, hashtags, and users.

In seed_tweets.py, change tweet object creation to reference updated names of cols in database.
Add UNIQUE=TRUE to tw_tweet_id, so that tweets cannot be duplicates.
Make relationship between hashtags (plural) and status (singular)
Add user_id column to reference User class (currently not in use)
WIP: handle exception if duplicate tweet ID is added to session
Database holds 6000 tweets and associated hashtags
Refactor load_tweets to try/except each add and commit for each status.

Because statuses are loaded one a time, if an exception occurs, I can rollback the single tweet and continue committing other tweets.
Rewrote relationship and foreign key in Hashtag class (child table of Status).

 - Changed relationship in Hashtag class to explicitly set foreign key to Status.tw_tweet.id
 - removed typo additional backref() in backref parameter of relationship
 - In seed_tweets, set hashtag.status_tw_id to status[id]
 - remove status_id column from Hashtag table. Foreign key is now statuses.tw_tweet_id
 - Recreated database, loaded 6000 tweets into Status and Hashtag table. Process took over 1 minute, probably due to indexing and multiple table additions, as well as committing to database after each tweet.
WIP - working on getting co-occurring hashtags as a classmethod of Hashtag
Removed contents of co-occurrence function, moved back to Hashtag class