Changed the definition of the Tweet.user_id column to be the same as User.id instead of User.twitter_uid.

In order to preserve the data, this needed to be done in three steps.
1. Rename the existing Twitter.user_id to Twitter.twitter_uid since that's what it is, also moved the FK with it.
2. Add a new Twitter.user_id column and use the information from User.id to populate it (using Twitter.twitter_uid as the join between the tables). Also add an FK to this new column.
3. Drop the old Twitter.twitter_uid since its redundant.

We also changed the Models to match the new associations.
Added two scopes to Twitter (home_timeline & user_timeline) that reproduce the equivalent timelines from the user's Twitter stream.
Made sure that the user is always his own friend as far as Twitter is concerned, otherwise the home_timeline doesn't include the user's own tweets.
Changed the threshold for auto-explain down to 0.1 seconds to be a little more aggressive.
We now reproduce both the user and home timelines using the helpers that we wrote earlier.
Added unsorted timeline scopes in addition to the existing sorted options. The default was changed to the unsorted version.
When updating friendships, kill off the current set, so that we properly update for unfollowed accounts.
We now show a pie chart with the top 10 people in the user's timeline.
Made the piechart "3d".
Added a bar chart with the same information. Also corrected the column names.