Add association table between tweets and hashtags (many to many)

Create hashtags relationship on Status class
Create association table statuses_hashtags to link twitter tweet id (from twitter) to a hashtag by hashtag id (database id)
Remove foreign key column from Hashtag - now only contains id and text column
Re-added back tw_user_id to Status class in order to get user id for any given tweet (since User class is not in use yet)
Remove separate load_hashtags func, add hashtags to tweet for each tweet, check against dictionary

Make hashtag text lowercase to avoid duplicates through capitalization
Create dictionary of hashtags to avoid adding duplicates
For each hashtag object in a hashtags list, add to each tweet's hashtag list
Through relationship/backref, session.add(tweet) also adds corresponding hashtag to session - don't have to add hashtag to session separately
Delete load_hashtags function
Updated classifer ipython notebook
Database contains tweets with hashtags attr through association table.

Also readded user twitter id column to tweets.