Refactor of ebooks.py (and fix a couple things)

* Fix issue #6 (https://github.com/tommeagher/heroku_ebooks/issues/6): If api.GetUserTimeline() returns an empty user_tweets list, grab_tweets now returns that empty list and 0 as max_id.
* Refactor: move all tweet construction into try_build_tweet(source_tweets). Return None if conditions fail, return a tweet when we find one.
* Remove loop around min.generate_sentence; couldn't figure out the point of it... seemed to build ten totally unique and valid sentences.
* Refactor: move tweet posting (both API and DEBUG implementations) into post_tweet(ebook_tweet).
* Add loop around try_build_tweet() call. If try_build_tweet returns something good, post it. Otherwise, keep trying (up to ten times) before giving up.