Setup one table to save all tweets to

- Copy tweets from existing table (home, mentions, @<user>, @<user/list>
etc.,) to new `tweets table
- Create `timelines` table to keep track of which timelines to store and
what ID they last saw; model object encapsulates logic for detmerining
the appropriate twitter API endpoint & parameters
- Remove code to alter tables to utf8mb4 collation -- copying tweets to
`tweets` has the same effect
Make use of timelines object in gettweets
Update store tweets function for only one table
Update importarchive for one table schema
Update query builder for one table schema
Cleanup and document config helper
Remove suggestion of using @me for mentions
Remove outdated clarification from setup
Using short array syntax now, require PHP >= 5.4
Update conversation class for single table schema