Work for Old and New Spreadsheets, fixes #13

Hi, me!

This makes `sheetdown` work for both old and new spreadsheets but changes the API -- **you now must use the full spreadsheet url instead of the key**.

There may be a querystring that works for both new and old and we may not have to do the work @maxogden's module does to figure out if a spreadsheet is old or new, but I haven't found that magical querystring yet so until then --