Twitter API implementation

Fixes issue #14

You must set the following environmental variables to make this work:
```
TWITTER_API_KEY
TWITTER_SECRET
```
Api works as follows

```

Full-text search:
/api/VERSION/twitter/search/:search-term/:limit

Show @username's tweets:
/api/VERSION/twitter/user/:username/:limit

Show tweets with #hashtag:
/api/VERSION/twitter/hashtag/:hashtag/:limit
```

All APIs allow an optional limit to the number of tweets returned.