Faster response to click on get-url button.

If I want to download a table of the entire database, the page takes a couple
of seconds to create the compressed url. This version just hashes the document
position into [key,id] and uses this (instead of Array.indexOf) to build the
index list.
Used 'rows' instead of re-using keys for no reason.
Introduced a bunch of spaces.

This is to show that the implicit use of [rows[k].key,rows[k].id].toString()
yields a signature compatible to documents[d].toString(). It is way more
elegant than the excessive use of JSON.parse/stringify from before, but more
intricate as well.
Beautified the bit where start_key and end_key are built.