Driver selection

This patch allows users to select the driver they want to use (effectively, if they want to force localStorage or choose between WebSQL or IndexedDB on Chrome), but will allow for future patches where something like "**Don't** use localStorage" (#29) is possible.

Note to @sole: I basically went through everything and started lowercasing `localforage` everywhere. The mixed case was not only confusing, but started to cause weird bugs that existed on Mac but not on Linux. I called the library "localForage" as an allusion to "localStorage", but I'm happy to ditch the "clever" (not really) naming convention if it means more clarity, and I think it's generally annoying to developers to expect them to use mixed-case filenames and library names (see: jQuery is `jquery.js), etc.