Initial refactor

Francis
Not sure this is quite production ready yet, but have tested on a few of our scrapers and it works fine. There's some functionality (e.g. auto-creation of indices) that I haven't yet duplicated, but the public api seems to work, and it's fairly fully tested. The SQLite stuff is all extracted into its own gem, so this gem is now just a wrapper around it. Have also added a few tweaks, such as the ability to set the location of the db (which is pretty useful if you've got several files that call the gem using the same Gemfile).

Comments welcome
Chris