Large overhaul

I've done quite a lot:-
 - Make it use `lens`
 - It now relies on `heist` (which is a dependency of `snap` anyway), which means you only have to write `<recaptcha-script/>` and `<recaptcha-div/>` in a heist template. At the cost of flexibility beyond `heist`, of course.
 - Reduce and update dependencies (http-conduit now has a more complete API in particular)
 - Put the example/test thing in its own module, so it's readable in the haddocks.
 - Throw "HttpException" if Google's API is somehow uncontactable (not 100% on this yet, but I figured this is quite "exceptional" behaviour and there is probably not much the snaplet or its user can really do to fix it in their Haskell program)
 - Arbitrarily bumped to "1.0" to signify widespread API breakage.

I'd be happy to take over maintainership of this - though I'm not sure how to best do so - or just to put it on Hackage under a different name, since the changes are quite large.