added memoize function and wrapped get_tweets with it
Added lettuce/splinter BDD test suite and rewrote '/' endpoint

- moved Tweet model to twitter.py
- added tweets.feature for BDD (lettuce)
- added steps.py (also for BDD)
- modified index.html to jive with refactored '/' endpoint:
- moved the post/get into one URL: '/'
minor feature dir reshuffling

currently broken: importing within lettuce