add button to get data with other extraction method; needs testing

As we discussed elsewhere, I added a button to toggle between spreadsheet and original extraction methods. 

This needs testing. It worked in a very basic case and I haven't tested any more. Y'all should give it a shot and see what happens before merging.

Assuming it works, this would be ready for a prerelease version, I think.

One little bug: right now, it caches the response by the coords and the extraction_method param -- on the first request, that's "guess" and on subsequent ones, it's either "spreadsheet" or "original". On the third click (i.e. the first one is whatever the heuristic guessed, second is the other option and third click is what the heuristic guessed again) we're regenerating the data, since the param sent to Tabula is different (e.g. it might be "spreadsheet" instead of "guess"). Thsi should be easy enough to fix, but it's late and I need to go to sleep.