[Bug] Changing Databases

So I wanted, and expect to be able to, do this:

```ruby
ScraperWiki.config = {db: 'some.db'}
ScraperWiki.select('* from swdata') # Gets data from some.db
ScraperWiki.config = {db: 'some_other.db'}
ScraperWiki.select('* from swdata') # Still gets data from some.db. Umm, whut?
```

but to my surprise the second query still gets data from the first one! This pull request allows the above behaviour, shouldn't change the API and, for bonus points:

* Actually runs the tests when you run `rake` - Travis will be happy!
* Allows us to read the configuration settings with `ScraperWiki.config`